The related concepts of paedomorphosis in the secondary xylem,
insular woodiness, and secondary woodiness are reviewed and evaluated in
order to clearly distinguish the phenomenon involved, and provide a firm
foundation for future research in this area. The theory of
paedomorphosis refers to the occurrence of certain juvenile xylem
characteristics, such as scalariform perforation plates and lateral wall
pitting, in the secondary xylem of shrubby, suffrutescent, pachycaulous,
and lianoid growth forms. Paedomorphic characteristics are often found
in insular woody species, a fact that has caused paedomorphosis to be
associated with secondary woodiness. The anatomy of the secondary xylem
in Xanthorhiza simplicissima (Ranunculaceae), Coreopsis gigantea
(Asteraceae), and Mahonia bealei (Berberidaceae) is described in order
to provide specific data for discussion. These species serve as test
cases for the presence of paedomorphosis, and the evolution of secondary
woodiness. The secondary xylem of all three species was found to have a
degree of paedomorphosis, with Coreopsis having the greatest number of
paedomorphic characteristics, Xanthorhiza having an intermediate number,
and Mahonia possessing only a single characteristic. Plotting the
occurrence of the character states woody and nonwoody on phylogenetic
trees containing these taxa shows that Coreopsis is secondarily woody,
while the ancestry of the other two species cannot be unambiguously
established. These results must, however, be considered preliminary as
the occurrence of secondary growth in many "herbaceous" plants
often goes unreported. Although paedomorphosis is often associated with
secondary woodiness, there are examples of paedomorphic wood in
primitively woody taxa. One conclusion is that the degree of
paedomorphosis may be a better indicator of the mechanical requirements
of the shoot then of its evolutionary history.

Much of the research on the secondary xylem and associated tissues
such as the vascular cambium has been skewed towards typical woody
dicotyledons, like trees. This focus is seen in many of the articles in
the IAWA (International Association of Wood Anatomists) Journal, and in
the website InsideWood (InsideWood, 2004-onwards), an online wood
anatomy information and image database. This orientation is likely due
to the sheer usefulness of, and commercial applications for, the
secondary xylem of these plants. Much research has been geared towards
understanding stem anatomy in order to advance various commercial
applications of wood (Panshin & de Zeeuw, 1970). Even the efforts of
Kribs (1935) to develop a broad classification of rays were affected by
this focus. Kribs' ray classification system was based on the study
of a collection of mostly typical dicotyledonous woods (Carlquist,
2001).

Despite the emphasis on typical woody dicotyledons, there has been
some interest in the secondary xylem of plants that are not typically
woody. For example, Arabidopsis has recently been used as a genetic
model for both primary and secondary xylem development (Chaffey et al.,
2002; Melzer et al., 2008; Nieminen et al., 2004). There has also been
increased interest in the secondary growth of so called herbaceous (1)
plants, and in the anomalous secondary growth of plants with cambial
variants (Carlquist, 2007; Rajput & Rao, 1998; Rajput & Rao,
1999; Schweingruber, 2006; Schweingruber, 2007a; Schweingruber &
Landolt, 2005 2008).

This interest in non-commercial species is, of course, not a recent
phenomenon. In the 19th century both Darwin and Wallace became
interested in insular species of plants and animals (Darwin, 1859;
Wallace, 1878). As part of this interest, they debated how woody plants
could appear on islands. This phenomenon was puzzling because
environmental conditions seemed to limit the successful dispersal of
woody plant seeds. How could plants with limited dispersal reach
islands? Such issues, as well as a general interest in the xylem anatomy
of insular plants and their relatives, led scientists such as Chrysler
(1937) and Carlquist (1962) to study secondary xylem with anatomical
characteristics different from those of typical dicotyledons. Interest
in these plants, many of them found on islands, led to the theory of
paedomorphosis in the secondary xylem (Carlquist, 1962; Chrysler, 1937).
The concept of secondary woodiness is also closely associated with
insular woodiness, and paedomorphosis.

Insular woodiness occurs when plants from predominantly herbaceous
groups have woody members that occur on islands (or in mountainous
equatorial areas). Secondary woodiness refers to woodiness that is
derived from herbaceous ancestors. Paedomorphosis in the secondary xylem
refers to a suite of anatomical characteristics, many of which are
normally associated only with the primary xylem in typical woody
dicotyledons. Paedomorphic features are most often found in less woody
plants that have shrubby, suffrutescent, pachycaulous, or lianoid growth
forms.

Paedomorphosis in the secondary xylem, and the related concepts of
insular and secondary woodiness, are explored in this paper. In addition
to a review of these concepts, particular attention is directed to how
the presence of secondary growth in herbaceous plants affects the
theory. We also consider the secondary xylem of three species,
Xanthorhiza simplicissima Marsh., Coreopsis gigantea (Kellogg) H.M.
Hall, and Mahonia bealei (Fortune) Carr. that serve as test cases for
the evolution of secondary woodiness, and the presence of
paedomorphosis. The secondary xylem of these species is examined for
characteristics of paedomorphosis, and their phylogenies are reviewed
for evidence of secondary woodiness.

III. Historical Review

A. Paedomorphosis, Insular Woodiness, and Secondary Woodiness

The theory of paedomorphosis in the secondary xylem, and the terms
insular and secondary woodiness are related concepts that are complex,
nuanced, and sometimes controversial. In addition (and unfortunately),
these concepts are not always well defined or used consistently in the
literature. Sometimes they are conflated with each other, even though
they are not identical, and have important differences. For instance, a
plant with certain anatomical characteristics in the secondary xylem may
be described as having paedomorphic wood. Based on this, some
researchers imply (or come perilously close to implying) that it must
have secondary woodiness. However, without phylogenetic knowledge of its
ancestors, a plant cannot be definitively said to be secondarily woody.
That decision must be made in the context of its phylogenetic
relationships (Carlquist, 1962). There is similar confusion about
insular woodiness. Since insular woodiness can originate from either
herbaceous ancestors or extinct woody ancestors, decisions about its
origin are best be made in a phylogenetic context (Kim et al., 1996).
Due to the inherent complexities of these concepts, they must be clearly
differentiated.

Even the concepts of "woody" and "woodiness,"
which are used much more often in the literature, are often ambiguous.
Although wood is generally understood to mean the secondary xylem of
gymnosperms and dicotyledons (Evert, 2006), when applied to growth forms
woodiness can mean a variety of things. For instance, a plant may have a
woody stem, a woody root system, or both (Isnard et al., 2003).
Woodiness in growth forms may also exist in varying degrees, from a
small amount to a very substantial amount. A suffrutescent stem like
that found in Descurainia tanacetifolia (Brassicaceae) has secondary
xylem only at its base, while the remainder of the stem is herbaceous
(Goodson et al., 2006). The common presence of secondary xylem in
annuals and herbaceous perennials (Bowers, 1996; Bowers & Mauseth,
2008; Schweingruber & Poschlod, 2005), is sometimes referred to as
wood (Chaffey et al., 2002; Melzer et al., 2008; Nieminen et al., 2004;
Ye, 2002). This can result in typically herbaceous plants possessing
wood, and perhaps even being termed woody (Melzer et al., 2008). At the
other extreme, much of the tissue in perennial arborescent plants like
Acer rubrum (Sapindaceae) is typical wood, produced over a number of
years. Thus, whether a particular growth form is woody or not is often a
matter of interpretation.

B. Paedomorphosis

1. Genesis of the Theory of Paedomorphosis

Paedomorphosis is one of the possible end results of heterochrony.
Heterochrony is broadly defined as the change in both the rate of
development of characters, and the timing of the appearance of these
characters during development (de Beer, 1930; Gould, 1977; McNamara,
1986). Variation in growth rates, as well as the duration and timing of
the growth period, affect the relationship between size and shape of
particular structures (McNamara, 1986). Organisms with different rates
of development may possess adaptive advantages, which may be selected
for, thus shaping evolution (McNamara, 1986; McNamara & McKinney,
2005). Heterochrony is analyzed by comparing ontogenies of related
organisms. Morphological comparisons of changes in shape and size have
to be used together with heterochronic time data (McKinney, 1988).

There are two different results of heterochrony: paedomorphosis and
peramorphosis. Paedomorphosis results when an organism passes through
fewer developmental stages than its ancestor, resulting in the adult
form of the organism possessing morphological characteristics that
occurred in the juveniles of its ancestor (McNamara, 1986).
Peramorphosis, on the other hand, results if the organism passes through
more developmental stages than its ancestor, leading to the adult form
that is not represented in the ancestor (McNamara, 1986). Both
paedomorphosis and peramorphosis can be achieved in more than one way.
For example, paedomorphosis can occur by the reduction of the rate of
morphological development through the juvenile growth stage (neoteny),
by early sexual maturation (progenesis), or by the delay of the
beginning of morphological development (post-displacement) (McNamara,
1986).

Paedomorphosis has been applied to the study of both animals and
plants. For instance, paedomorphosis has been studied in the ammonite
Ptvtacanthoceras tuberculatum to investigate the evolution of its small
shell (Landman, 1988). In limpets, the shell form of subclade
Patellogastropoda was also achieved through paedomorphosis (Linberg,
1988). Although (Carlquist, 1962) is best known for applying the theory
to plant anatomy, the concept was discussed in the literature much
earlier than his seminal work. Chrysler's (1937) study of Zamia
(Cycadaceae) provides an early glimpse of the ideas that Carlquist would
later refine.

When Chrysler (1937) investigated xylem taken from a mature
specimen of Zamia floridana, he found tracheids with scalariform lateral
wall pitting throughout the xylem. This is in contrast with the other
genera in the Cycadales, which have either only tracheids with bordered
pits, or both tracheids with scalariform pitting (tracheids near the
pith) and tracheids with bordered pits (tracheids in the later formed
xylem). In the latter case, pitting changes from scalariform in the
earliest formed xylem, to bordered pits in the more mature xylem
(Chrysler, 1937).

Since Zamiafloridana has a tuberous stem, or caudex, Chrysler also
investigated pitting in two species which possess well defined trunks:
Z. pseudoparasitica and Z tuerckheimii. The xylem of a mature specimen
of Z. pseudoparasitica showed a pattern of tracheids with scalariform
lateral wall pitting close to the pith, tracheids with transitional
lateral wall pitting further out in the stem, and tracheids with
circular bordered pits closer to the phloem. The trunk of a young
specimen of Z tuerckheimii revealed only tracheids with scalariform
lateral wall pitting (Chrysler, 1937).

These findings suggest that tracheids with scalariform lateral wall
pitting are normally found only in immature cycads, or in the earliest
xylem of more mature cycads. Based on these observations, Chrysler
(1937) concluded that the tuberous species Z floridana is a persistent
juvenile with respect to its xylem. It produces only immature xylem
throughout its life. The presence of only tracheids with scalariform
wall pitting indicates an arrested development in the xylem. Although he
did not term it paedomorphosis, his observations are in essential
agreement with the theory later formulated by Carlquist (1962).

Carlquist (1962) developed the full theory of paedomorphosis in the
secondary xylem. The theory is an effort to explain why the anatomy of
the secondary xylem of some less woody plants with shrubby, herbaceous,
or lianoid forms does not fit the broad anatomical trends seen in the
secondary xylem of typical woody dicotyledons. His hypothesis is that
the anatomical changes seen in these plants are due to paedomorphosis.
In this context, paedomorphosis means that certain features of the
primary xylem are also found in the secondary xylem. Thus, the secondary
xylem of plants with paedomorphic wood shows anatomical characteristics
usually associated with the primary xylem of typical woody dicotyledons.
Paedomorphic secondary xylem is either permanently juvenile, or else
loses its juvenile characteristics slowly as it grows. In less woody
plants, paedomorphic secondary xylem retains juvenile characteristics
found in the primary xylem because the plants are either moving
evolutionarily towards, or away from, true woodiness (Carlquist, 1962).

2. Bailey's Refugium Theory and Major Trends in Xylem
Evolution

In formulating his theory of paedomorphosis, Carlquist (1962) drew
on Bailey's (1944) refugium theory (Carlquist, 2009). This theory
states that there is an evolutionary lag in the development of the
tracheary elements in the primary xylem, compared to those in the
secondary xylem. According to the refugium theory, vessels first
originated in the secondary xylem, replacing tracheids. Over time,
vessels in the secondary xylem became ever more specialized. Increasing
specialization was evident in a directional series of structural changes
in the elements. The changes included changes in lateral wall deposition
patterns, modification of the vessel end walls, and a decrease in vessel
element length. These changes did not occur as quickly in the primary
xylem, so that eventually the vessels of the primary xylem were less
specialized than those of the secondary xylem. As a result of this
evolutionary lag, the primary xylem retains more primitive tracheary
characteristics than the secondary xylem. Thus, the primary xylem serves
as a refugium for these more primitive characteristics (Bailey, 1944).

In developing the refugium theory, Bailey drew on the major trends
of tracheary element evolution that he helped formulate during the first
half of the 20th century (Bailey, 1944; Bailey & Tupper, 1918;
Frost, 1930a, b, 1931). These trends eventually became known as the
Baileyan trends. They were intended to indicate the general direction of
evolution in tracheary elements. Later, other researchers used the
Baileyan trends to establish evolutionary direction in other xylary cell
and tissue types, such as rays (Kribs, 1935) and wood parenchyma (Kribs,
1937).

Several of the major Baileyan trends concern tracheary elements,
especially vessels (Bailey, 1944; Bailey & Tupper, 1918; Frost,
1930a, b, 1931). The vessels which occur in plants found earliest in the
fossil record have annular or helical lateral wall deposition patterns.
Because these patterns are found earlier in the fossil record, they were
considered primitive, or less advanced (see Wagner, 1969 for a critique
of this idea). If these patterns were found in an extant plant, they
would be considered primitive. Slightly later in the fossil record
scalariform patterns appear. Finally the most advanced, or specialized,
pattern appears in the record: vessels with pitted lateral walls. Within
this category, vessels evolved from possessing opposite to alternate
pitting.

The changes from annular to pitted lateral walls were paralleled by
a series of changes in the vessel end walls. Vessels that appear earlier
in the fossil record have scalariform perforation plates (bars across
the opening of the end wall). More advanced vessels have fewer bars,
until eventually no bars are present (simple perforation plates). The
vessel elements also become shorter, and develop more transverse (less
angled) end walls. Unlike the end walls of vessels with scalariform
perforation palates, transverse end walls do not overlap with those of
neighboring vessel elements (Bailey, 1944; Frost, 1930a, b, 1931).

Trends in the distribution of vessels were also established
(Gilbert, 1940). Diffuse porous woods, where the vessel diameters remain
relatively constant in a growth ring, were found to be less advanced.
The ring porous condition, which consists of clearly delineated areas
with larger diameter vessels (earlywood) alternating with areas of
smaller vessels (latewood) was more advanced (Gilbert, 1940).

Another major trend involved the appearance and evolution of
fibers. Fibers, like vessels, are absent from the earliest fossils, and
so must also have evolved from tracheids. Unlike vessels, which are
specialized for water conduction, fibers are adapted to a specialized
support function. The less advanced form is the fiber tracheid. Fiber
tracheids are intermediate in form between tracheids and true fibers.
They are considered imperforate tracheary elements like tracheids, so
they are capable of transporting water only through their porous pit
membranes. They also have bordered lateral wall pitting, like tracheids.
The more advanced form is the libriform fiber, which possesses simple
pits (pits with reduced or absent borders) (Evert, 2006).

Kribs examined ray parenchyma (Kribs, 1935) and axial wood
parenchyma (Kribs, 1937) to determine their evolutionary trends. In the
case of axial parenchyma, he concluded that its absence is the most
primitive condition. Diffuse axial parenchyma (single strands or cells
scattered among the xylary fibers) is advanced. Metatracheal parenchyma
(axial parenchyma in concentric rings parallel to the cambium, but
independent of the vessels) is more advanced. Vasicentric parenchyma
(parenchyma surrounding the vessels) is the most advanced type of axial
wood parenchyma (Kribs, 1937). In the case of ray parenchyma,
heterogeneous rays possessing both upright and procumbent cells are most
primitive. More advanced rays have increasing levels of cell
homogeneity. Homogeneous rays, consisting of only ray cells with
procumbent orientation, are the most advanced (Kribs, 1935).

Kribs' ray classification system was based on the study of a
wood collection that contains mostly trees. As a result, it does not
include categories for ray types found in less woody growth forms such
as woody herbs (herbaceous plants that have evolved the ability to
produce at least some limited secondary xylem) (Carlquist, 1974), or
rosette trees. To correct this problem, Carlquist (2001) modified the
system to include paedomorphic ray types found in less woody plants.
These additions to ray types include Paedomorphic Type I, Paedomorphic
Type II, and Paedomorphic Type III (Carlquist, 2001).

The Paedomorphic Type I category contains both multiseriate and
uniseriate rays. Upright ray cells predominate in the multiseriate rays,
with any procumbent cells restricted to the multiseriate portion of the
ray. The uniseriate rays have only upright cells. Examples of plants
with Paedomorphic Type I rays include Verbesina spp. (Asteraceae), and
some species of Euphorbia (Euphorbiaceae) (Carlquist, 2001).

Paedomorphic Type II rays are almost exclusively multiseriate with
solely or predominantly upright cells. Any procumbent cells are found in
the multiseriate portion of the rays. Although uniseriate rays may occur
in this type, they are uncommon. Geranium tridens (Geraniaceae) and
Ardisia brackenridgei (Myrsinaceae) contain Paedomorphic Type II rays
(Carlquist, 2001).

Paedomorphic Type Ill rays are uniseriate with exclusively upright
cells. No multiseriate rays are found in this type. Plants with
Paedomorphic Type III rays include Corema conradii and Dracophyllum
acerosum (Ericaceae) (Carlquist, 2001).

For the most part, the Baileyan trends were initially treated as
irreversible evolutionary trends. However, the xylem of a single species
did not necessarily have only primitive or advanced characteristics.
Depending on the level of functional organization examined, there could
be a mix of the trends. Within even one cell type, vessel elements for
example, one characteristic might be more advanced (simple perforation
plates) while another might be less advanced (scalariform lateral wall
pitting). If the secondary xylem had a preponderance of cell types with
primitive or specialized characteristics, it was treated as structurally
primitive or advanced (Bailey, 1944). However, in specific plants one
set of characteristics could be more, or less, advanced than another
(Bailey, 1944). For example, Pentaphragma decurrens (Pentaphragmataceae)
has vessels with primitive characteristics, while the fact that it lacks
rays (or has delayed ray development) is a more specialized feature
(Carlquist, 1997).

All of the major trends in xylem evolution were discovered by use
of a few simple methods. First, the fossil record was used to determine
the specialization of a feature. For example, if tracheids appeared in
the fossil record before vessels, then the tracheids were less
specialized, or more primitive, than the vessels (Bailey, 1944; Frost,
1930a, b, 1931). Vessels would, in turn, be considered more advanced, or
specialized, than tracheids. Second, statistical correlations were used
to establish relationships between trends. A characteristic or cell type
recognized as primitive due to its place in the fossil record would be
compared to other characteristics whose level of advancement was not
known. If a given characteristic had a high correlation with the
characteristic already recognized as primitive, then it too was
primitive.

For example, early researchers such as Frost (1930a, b, 1931)
recognized that tracheids appeared in the fossil record before vessels.
Thus, they were considered primitive. It also seemed reasonable that
vessels were derived from tracheids, since vessels are found later in
the fossil record, and have the same types of lateral wall pitting. In
addition they both fulfill the same function, conducting water. Since
tracheids are longer than vessel elements, any characteristics
correlated with longer vessel element lengths were also considered
primitive, and characteristics associated with shorter vessel elements
were more advanced (Bailey, 1944; Frost, 1930a, b, 1931 ). The use of
statistical correlations was extended beyond vessels to other xylem
components, such as rays and xylary parenchyma. Patterns of parenchyma
distribution in the xylem as well as ray composition type were both
correlated with vessel characteristics to determine primitive and
advanced states (Gilbert, 1940; Kribs, 1935, 1937).

The most primitive states of the Baileyan trends tend to be found
in the primary xylem of typical dicotyledonous woody plants. This region
usually contains longer vessel elements with scalariform lateral wall
pitting with wide lateral wall pit apertures, thin walled fibers with
wide lumens (or no fibers), and tall primary rays (interfascicular
regions of the primary plant body that are continuous with rays in the
secondary xylem) with erect cells (Carlquist, 2001; Carlquist, 2009).

Bailey's (1944) refugium theory was an attempt to explain why
the primary xylem retains these primitive features. According to this
theory vessels evolved first and became more specialized in the
secondary xylem. Evolution and specialization then proceeded to the late
primary xylem (the metaxylem, which matures after elongation of the
primary plant body is completed), and then to the early primary xylem
(the protoxylem, which matures in actively elongating tissues of the
primary plant body). As a result of these events, the primary xylem is
left with more primitive features than the secondary xylem (Bailey,
1944; Esau, 1977).

These trends, and this explanation of their occurrence, set the
stage for Carlquist's theory of paedomorphosis (Carlquist, 1962).
Carlquist found that many of the more primitive xylem characteristics
were expressed in the secondary xylem of herbs, woody herbs, rosette
trees/shrubs, and stem succulents that were included in his study. Later
studies confirmed these tendencies, and led to the description of new
characters associated with paedomorphosis (Carlquist, 1974, 1983, 1989,
1997; Carlquist, 2001; Carlquist, 2003, 2009; Lens et al., 2005a). The
paedomorphic characteristics described in this research include
scalariform or pseudoscalariform lateral wall pitting on the vessel
elements; vessel elements with simple (most common) or scalariform (less
common) perforation plates; wide, thin-walled fibers and the
predominance of libriform fibers, or parenchyma cells replacing fibers;
either the absence of rays or delayed ray development; and rays, when
present, consisting of mostly upright, or square, cells (Carlquist,
1962; Carlquist, 2001; Carlquist, 2009). Tracheids are usually absent
from paedomorphic woods. Carlquist (1962) also noted that paedomorphic
woods produce decreasing, or stable, vessel element lengths as the
secondary xylem ages (Fig. 1).

3. Paedomorphic Characters' of the Secondary Xylem

Many of the paedomorphic characteristics can be expressed
independently of each other, so that not all paedomorphic woods have all
of the paedomorphic characteristics. The lack of linkage between these
characteristics can lead to one or two paedomorphic features occurring
in xylem that otherwise has more typical characteristics (Carlquist,
1962). Thus, there is a continuous range of paedomorphic woods from
those expressing only one or two paedomorphic characteristics, to those
with the full suite of characters described above.

[FIGURE 1 OMITTED]

It is also important to remember that not all of the characters
initially mentioned in association with paedomorphosis are the juvenile
states found in the primary xylem. While the term
"paedomorphosis" leads one to expect that only juvenile states
will be found in paedomorphic wood, the original concept of
paedomorphosis was more subtle. While certain paedomorphic
characteristics, like vessels with scalariform lateral wall pitting, are
associated with the primary xylem (at least in typically woody species),
other paedomorphic characteristics, such as the presence of libriform
fibers and simple perforation plates, are not. These features were
initially included in lists of paedomorphic characters because their
occurrence was thought to be correlated with other paedomorphic
characteristics, which are truly juvenile (Carlquist, 1962; Carlquist,
2001). However, the widespread occurrence of libriform fibers in groups
such as the Asteraceae (Carlquist, 1966), and the common occurrence of
simple perforation plates in the secondary xylem of many species
suggests that these correlations are only apparent, and may be due to
limited sampling (Table 1) (Schweingruber, 2006; Schweingmber, 2007a).
That fact that libriform fibers and simple perforation plates sometimes
co-occur with paedomorphic characters does not mean that they are
correlated with them across a wide range of taxa. This has been
recognized in recent treatments of paedomorphosis, which claim no
relationship between fiber type and paedomorphosis (Carlquist, 2009).
For instance, Lens et al. (2007) used only the presence of extremely
short vessel elements, the occurrence of wide scalariform intervessel
pits, and absence of rays (or the presence of exclusively upright cells
in multiseriate rays), as indicators of paedomorphosis.

Perhaps the most characteristic feature of paedomorphic woods is
the age-on-length curve that describes changes in vessel element length
across the secondary xylem (Fig. 1) (Carlquist, 1962; Carlquist, 2001;
Carlquist, 2009). Typical paedomorphic wood has either vessel elements
that decrease in length as the secondary xylem grows (typified by
Talinum guadalupense, Portulacaceae; Fig. 1), or has vessel elements
whose lengths initially decrease, but then remain roughly constant
through the remainder of the secondary xylem (typified by Macropiper
excelsum, Piperaceae; Fig. 1) (Carlquist, 1962; Carlquist, 2001;
Carlquist, 2009). When vessel element lengths in the secondary xylem are
graphed in age-on-length curves, the former case shows a negatively
sloped curve, while the latter shows a nearly flat curve (Fig. 1)
(Carlquist, 1962).

These two patterns of vessel element length contrast with the
typical pattern seen in normal woody dicotyledons, such as Eriobotrya
japonica (Rosaceae) (Fig. 1). In E. japonica the earliest formed vessel
elements of the secondary xylem continue the decrease in length seen in
the primary xylem. However, at some point fairly early in the process of
secondary growth, the vessel elements begin to increase in length. As
secondary growth continues, vessel element lengths level off, and
eventually may even decline somewhat. The paedomorphic age-on-length
curves described above are similar to the descending portion (found
mainly in the primary xylem) of age-on-length curves for typical woody
dicotyledons (Carlquist, 1962, 2009).

Nearly flat age-on-length curves have been reported for vessel
elements in several woody annuals of the Asteraceae, such as Dicoria
canescens, Gnaphalium californicum, and Helianthus annuus (Carlquist,
1962). They are also found in stem succulents like Cereus gigantea
(Cactaceae) and Senecio praecox (Asteraceae), rosette trees like Carica
candamarcensis (Caricaceae), and woody herbs like Sonchus leptocephalus
(Asteraceae) (Carlquist, 1962).

Negatively sloped age-on-length curves are also reported in some
woody annuals of the Asteraceae, including Ambrosia hispida,
Blepharizonia plumosa, Cirsium californicum, and Madia sativa. They also
occur in the stem succulents Begonia coccinea (Begoniaceae) and
Brighamia insignis (Campanulaceae), in the rosette tree Scaevola
kauaiensis (Goodeniaceae) (Carlquist, 1962), in Impatiens arguta, I.
niamniamensis (Balsaminaceae) (Lens et al., 2005a), and in all species
in Corema and Empetrum (both Ericaceae) (Carlquist, 1989).

The developmental mechanics that lead to negative age-on-length
curves are straight forward. In a typical woody dicotyledon, transverse
divisions in fusiform cambial initials increase as the primary xylem is
formed so that vessel element length gradually decreases. During the
transition from the primary to secondary xylem, the number of transverse
divisions is reduced and intrusive growth of the fusiform cambial
initials causes cell elongation so that the vessel elements become
longer. Eventually, the number of transverse divisions increases and
intrusive growth declines, so that vessel element length plateaus, then
drops slightly. In paedomorphic wood the transverse divisions in the
fusiform cambial initials continue, and there is little cell elongation.
If these patterns are maintained indefinitely, a plot of vessel element
lengths will give a negatively sloped age-on-length curve. If they
continue for a shorter time, then the age-on-length curve will flatten
out after the initial brief negative slope (Carlquist, 2001).

Whether the negatively sloped or flat age-on-length curves found in
paedomorphic woods offer any functional advantage is unknown. It may
simply be that since many paedomorphic woods are found in short, rosette
shrubs that have thick parenchyma-filled stems, the increased mechanical
strength offered by longer vessel elements is less important, and is not
selected for. Longer vessels are stronger because the areas where the
vessel elements abut are mechanically weaker than the lateral walls of
the vessel elements. Thus, for a given vessel length, longer vessel
elements provide fewer potential weak points than shorter vessel
elements. In short, rosette shrubs, the parenchyma may provide
sufficient mechanical strength for the growth form (Carlquist, 2001).

Scalariform (or scalariform-transitional) and pseudoscalariform
lateral wall pitting between vessels, and between vessels and parenchyma
cells, is another anatomical feature still considered indicative of
paedomorphic secondary xylem. Scalariform lateral wall pitting occurs in
vessels with flattened sides (facets) where two vessels meet, or more
commonly, where vessels contact rays. The width of the pits corresponds
to the width of the flattened side of the vessels. In pseudoscalariform
lateral wall pitting the pits are either shorter, or longer, than the
flattened vessel sides. When they are longer, they extend part of the
way around the circumference of the vessel (Carlquist, 2001). When they
are shorter, the pits do not reach to the sides of the flattened walls.
Pseudoscalariform lateral wall pitting is usually found only in plants
with paedomorphosis (Carlquist, 1962; Carlquist, 2001). Begonia
coccinea, Cereus gigantea, Cariea candamarcensis, Chimantaea mirabilis,
Espeletia hartwegiana, Phoenicoseris regia, and Senecio praecox (all
Asteraceae), as well as Pentaphragma decurrens (Pentaphragmataceae), and
Impatiens niamniamensis (Carlquist, 1962, 1997; Lens et al., 2005a) are
examples of plants with scalariform wall pitting, and paedomorphic
woods. Except for Chimantaea mirabilis, all of these plants also have
tall pit apertures. Tall pit apertures are another characteristic of
paedomorphic woods (Carlquist, 1962, 1997; Lens et al., 2005a). Vessels
with scalariform or pseudoscalariform lateral wall pitting have less
cell wall strength due to the larger size of the pit apertures, and are
common in highly parenchymatized woods. Since the parenchyma in these
plants provides mechanical strength through cell turgor, it may be that
there is no negative selection against these paedomorphic
characteristics (Carlquist, 2001).

Most plants with paedomorphic wood have vessels with simple
perforation plates, though the presence of simple perforation plates is
not in itself an indication of paedomorphosis (Carlquist, 2001;
Carlquist, 2009). Simple perforation plates occur in paedomorphic wood
that has been identified as such by other characters (scalariform
lateral wall pitting, etc.), but by themselves they do not contribute to
this determination. Although Carlquist (1962) initially listed them as a
characteristic of paedomorphic wood, more recent treatments have
recognized the presence of simple perforation plates as an advanced
character (Carlquist, 2009).

The occurrence of scalariform perforation plates in the secondary
xylem is a more reliable indication of paedomorphosis. In the families
Campanulaceae, Pentaphragmataceae, Valerianaceae, and Asteraceae
scalariform perforation plates are occasionally found in the secondary
xylem. In the genera where they occur, they are an indication of
paedomorphosis since they are more common in the primary xylem and early
secondary xylem (Carlquist, 1983, 1997). For example, scalariform
perforation plates are found in the vessel elements of three species of
Pentaphragma (P. decurrens, P. horsfieldii, and P. sp.), all of which
have paedomorphic wood (Carlquist, 1997). In the Campanulaceae and
Pentaphragmataceae the plants that have scalariform perforation plates
are mesophytes living in moist forest understories. In these
environments transpiration rates are likely to be lower, so increased
rates of hydraulic flow promoted by simple perforation plates are not
critical. As a result, scalariform perforation plates may persist
because they are not selected against (Carlquist, 1983, 1997).

In many plants with paedomorphic secondary xylem, axial parenchyma
is usually extremely abundant, and may be the only axial component of
the secondary xylem apart from vessels. This is often a continuation of
the primary xylem structure in these plants, which is heavily
parenchymatized and has few, if any fibers (Carlquist, 1962; Carlquist,
2001). In contrast, typical woody dicotyledons have secondary xylem with
fibers and/or tracheids in addition to vessels and axial parenchyma, as
well as heavily parenchymatized primary xylem (Carlquist, 1962).
Abundant axial parenchyma is found in the paedomorphic wood of plants
such as Impatiens niamniamensis (Balsaminaceae) (Lens et al., 2005a),
Carica candamarcensis (Caricaceae), Wunderlichia mirabilis (Asteraceae),
Scaevola kauaiensis (Goodeniaceae), Brighamia insignis (Campanulaceae),
and Talinum guadalupense (Portulacaceae) (Carlquist, 1962). Recently,
however, the use of abundant axial parenchyma as an indication of
paedomorphic secondary xylem has been called into question (Carlquist,
2009). While its physiological activities (the storage of starch and the
conduction of sugars) are linked to those of the rays (the radial
conduction of sugars), its evolution has not been directly linked to ray
evolution. Unlike rays, the distribution of axial parenchyma does not
show heterochronic changes (Carlquist, 2009).

Some paedomorphic woods have relatively wide, thin-walled libriform
fibers, or fiber tracheids (Carlquist, 1962; Carlquist, 2001). Tracheids
are uncommon in paedomorphic wood (Carlquist, 2001). Impatiens arguta
(Lens et al., 2005a), Begonia coccinea (Begoniaceae), Cereus gigantea
(Cactaceae), Senecio praecox and Sonchus leptocephalus (both Asteraceae)
all have thin-walled, wide libriform fibers (Carlquist, 1962), but these
types of fibers are also common in many less woody plants, especially
those of the Asteraceae (Carlquist, 1966), and are not currently
considered indications of paedomorphosis (Carlquist, 2009). Fiber
tracheids are found in the branching canes of Chloranthus erectus
(Chloranthaceae), a species reported to have other paedomorphic
characteristics such as upright ray cells and scalariform perforation
plates (Carlquist, 1992). They also occur in the rayless, paedomorphic
wood of Pentaphragma decurrens and P. horsfieldii (Carlquist, 1992), but
again are not currently considered primary indications of
paedomorphosis.

Paedomorphic secondary xylem is also distinguished by the presence
of rays with exclusively, or at least predominantly, upright (or square)
ray parenchyma cells, with few or no procumbent cells (Carlquist, 1962,
1970, 1983, 1989; Carlquist, 2001; Carlquist, 2009). However, in some
species with paedomorphic wood a few rows of procumbent cells may occur
in the middle of larger rays consisting of largely upright ray cells
(Carlquist, 1962). The predominance of upright ray cells in paedomorphic
wood is in contrast to most typical woody dicotyledons, where procumbent
ray cells are usually more common (Mauseth, 1988). In paedomorphic wood,
the interfascicular regions of the primary xylem (primary or pith rays)
often contain mainly upright parenchyma cells, and this phenomenon
continues into the rays of the secondary xylem (Carlquist, 1962, 2009).

The orientation of ray cells (in radial sections) is related to the
direction of travel of solutes in the xylem: procumbent ray cells
conduct solutes radially, while upright ray cells conduct solutes
vertically, as well as some radial transport (Carlquist, 2009).

The increased number of upright ray cells in paedomorphic wood
results from a decrease in the transverse divisions of the ray initials
(Carlquist, 2001). This phenomenon is seen in Impatiens niamniamensis
(Balsaminaceae) (Lens et al., 2005a), Vernonia salviniae and
Wunderlichia mirabilis (Asteraceae), Brighamia insignis Trematolobelia
macrostachys and Delissea undulata (Campanulaceae), Lobelia gibberoa
(Lobeliaceae), and in some members of Polygonaceae such as Antigonon
leptopus (Carlquist, 1962, 2003).

When they are present, paedomorphic rays are frequently high and
wide (Carlquist, 1962, 2009; Lens et al., 2005a; Lens et al., 2007).
These types of rays are considered paedomorphic because, in typical
woody dicotyledons, the primary rays are often higher and wider than the
rays of the secondary xylem. As a result, high and wide rays that
continue from the primary into the secondary xylem are a juvenile
feature, and have been used as an indication of paedomorphic wood
(Carlquist, 1962, 2009).

Raylessness is also a characteristic of paedomorphic wood
(Carlquist, 1970; Carlquist, 2001; Carlquist, 2009). The rayless
condition can persist for the life of the plant, as seen in Impatiens
arguta (Lens et al., 2005a), and in Plantago princeps (Plantaginaceae)
(Carlquist, 2001), Viola trachelifolia (Violaceae) (Carlquist, 2001),
Stylidium (Stylidiaceae), Besleria (Gesneriaceae), and Aeonium
(Crassulaceae) (Carlquist, 2001). In other cases, transverse divisions
in what will become ray initials are delayed, but divisions eventually
begin and the plant develops rays as the amount of secondary xylem
increases. This happens in Cyrtandra c.f. propinqua, and Ixanthus
viscosus (both Gesneriaceae; Carlquist, 1974), and in at least one
species of Pentaphragma (Carlquist, 1997; Carlquist, 2001).

In some species that lack rays (e.g., Plantago princeps), some
fiber tracheids appear to be derived from areas of the cambium that
would normally produce rays (Carlquist, 1970). The substitution of fiber
tracheids for ray cells in rayless species may be a means of providing
additional structural support for these stems. In some woody species
with short lived stems, fiber tracheids may provide adequate support
given the limited structural needs. As a result, raylessness may be
positively selected for in these species (Carlquist, 2001).

The number of paedomorphic characteristics expressed in the
secondary xylem is a measure of the degree of paedomorphosis present in
a taxon. For example, the only paedomorphic characters found in the
three Pentaphragma ssp. investigated by Carlquist (1997) are raylessness
(or perhaps delayed ray development), vessel elements with scalarifonn
lateral wall pitting, and the occurrence of occasional scalariform
perforation plates. In other taxa, several paedomorphic characteristics
occur in the secondary xylem. For instance, Foeniculum vulgare
(Apiaceae) has a negatively sloped age-on-length curve, vessel elements
with scalariform lateral wall pitting and wide pit apertures, and mostly
erect ray cells (Carlquist, 1962). Though both of these species have
paedomorphic secondary xylem, E vulgare shows a greater degree of
paedomorphosis.

Although paedomorphic anatomical characteristics are indications of
juvenile wood, by themselves they should not be used to make decisions
about whether the ancestry of a plant is herbaceous or woody. Although
they may provide circumstantial evidence, they are first and foremost
structural characters that must be interpreted in some theoretical
framework. They cannot be conclusive by themselves because
paedomorphosis can exist in plants with either woody or herbaceous
ancestors (Carlquist, 1962). Questions of ancestry should be resolved in
a phylogenetic context, as discussed more fully below.

C. Insular Woodiness

1. Examples of Insular Woodiness

Many plants with paedomorphic wood are found on islands. Insular
woodiness refers to the woodiness of island plants that occur in
predominantly herbaceous groups. In this context, insular means oceanic
islands, continental islands, or mountainous areas near the equator.
Equatorial highlands share certain characteristics with islands, such as
relatively uniform yearly climates. In a sense they are
"islands" of isolated flora and fauna that are distinctly
different from that of the surrounding lowlands. Examples of equatorial
highlands include the New Guinea highlands, the highlands of Colombia
and Venezuela, and Mt. Kenya in eastern Africa (Carlquist, 1974).

Oceanic islands differ from continental islands in geologic origin,
age, and their source of biota. Oceanic islands such as the Hawaiian and
Canary Islands are of volcanic origin, and are geologically much younger
than continental islands. Often they are more isolated than continental
islands (but there are exceptions such as the Canaries, which are only
322 km from continental Africa) (Givnish, 1982). The entire flora and
fauna of oceanic islands are necessarily derived from colonization from
the mainland (Hubbell, 1968). Continental islands like New Zealand,
Madagascar, and the California Islands (except perhaps Guadalupe) were
once attached to continents, but were separated by rising sea levels
(Hubbell, 1968; Thorne, 1969). As a result of their geologic origins,
they are typically much older, and are frequently less isolated than
oceanic islands (Thorne, 1969). During the time they were joined to the
continents they shared the same flora and fauna. As a result, at least
part of their biota is derived from direct continental contact (Hubbell,
1968).

Plants with insular woodiness have a wide variety of growth forms.
Some are rosette herbs that have extended vegetative periods. This
extended growth period produces elongated stems that eventually flower.
Examples of plants with this growth form can be found on Hawaii
(Wilkesia gymnoxiphium, Asteraceae), the Canary Islands (Echium
pininana, Boraginaceae), and the Kenyan plateau (Dendrosenecio
keniodendron, Asteraceae). Sometimes plants with rosette growth forms
develop lateral branches at the base, which converts the monocarpic
rosette into a shrub. The shrub-like growth form of Stephanomeria
blairii (Asteraceae; San Clemente Island, CA) is an example of this
growth form (Carlquist, 1974).

Other insular woody plants produce lateral branches from nodes
beneath the inflorescences to produce a candelabrum-like growth form, as
seen in some species of Echium (Boraginaceae; Macaronesia). Another
example of a plant with this growth form is Euphorbia candelabrum
(Euphorbiaceae; Kenyan plateau). Axillary inflorescences that continue
to grow indefinitely may occur in some woody growth forms, like Plantago
robusta (Plantaginaceae; St. Helena), and P. arborescens (Canary
Islands) (Carlquist, 1974). Some shrubby genera have increased in height
on islands. One example of this phenomenon is the genus Sarcopygme
(Rubiaceae, Borneo) (Carlquist, 1974).

Woody arborescent plants with erect, bloated trunks and thick main
branches (the "bottle tree" or pachycaulous habit) also occur
on islands. Dendrosicyos socotrana (Cucurbitaceae) from Socotra Island
off the coast of Yemen is an example of this type of plant (Olson,
2003). Other species are woody, tree-like, and grow in very dry
conditions. Sida eggersii (Malvaceae; West Indies) is an example of one
of these species (Carlquist, 1974). Other examples are Senecio
vaccinioides (Asteraceae) and Hesperomeles ferruginea (Rosaceae) from
the paramos of the Andes (Carlquist, 1974).

2. Relictual and Secondary Insular Woodiness

There are two broad hypotheses for the origin of insular woodiness.
One hypothesis is that plants with insular woodiness are relicts,
descendents of woody continental species that are now extinct
(Carlquist, 1974; Mort et al., 2004). In some cases, where the woody
island plants are nearly herbaceous, they may have evolved from woody
ancestors, reducing the amount of secondary growth in the process
(Carlquist, 1974). This type of relictual woodiness has been described
in taxa from Macaronesia (Barber et al., 2002; Goodson et al., 2006). In
Macaronesia, at least some woody taxa are descended from woody
continental ancestors that were once abundant, but were apparently
driven to extinction either in Europe during the Pleistocene glaciation,
or in Africa by desertification. In this scenario, insular woodiness is
an ancestral character state, or plesiomorphy.

The second hypothesis is that plants with insular woodiness evolved
from herbaceous continental ancestors (Barber et al., 2002; Carlquist,
1974; Goodson et al., 2006; Mort et al., 2002). In this scenario,
insular woodiness is a derived character state, or apomorphy. Insular
woodiness of this type is also a form of secondary woodiness. Although
relictual insular woodiness has been found in all seven species of
Descurainia (Brassicaceae, Canary Islands) (Goodson et al., 2006), and
for all fourteen species of Pericallis (Asteraceae, Macaronesia)
(Swenson & Manns, 2003), the evolution of secondary woodiness on
islands is more common (Mort et al., 2002).

Distinguishing between a relictual and secondary origin of insular
woodiness can be difficult. Many island plants have diverged so greatly
from mainland ancestors that it is difficult to place them in
phylogenetic analyses (Givnish, 1998; Kim et al., 1996). Typical
problems involve determining the source of the original island
colonizers, determining when the colonization took place, estimating the
number of introductions, and determining the ancestors of the island
genera (Givnish, 1998). Although morphological data has often been used
to address these problems, it is sometimes difficult to tell whether a
shared character state is due to common ancestry, or convergent
evolution (Givnish, 1998). To address this problem, molecular sequences
have increasingly been used, sometimes in conjunction with morphological
data, to determine the origin of insular woodiness.

Some examples will illustrate how these studies have been
conducted. Goodson et al. (2006) used sequences from seven non-coding
chloroplast regions (cpDNA; rps16 intron, trnDGUCtrnEUUC, trnEUUC-trn
TGGU, psbZ-trnfMCAU, rpoB-trnCGCA, ndhFrp132, ndhC-trnVUAC) along with
internal transcribed spacers (ITS) of the ribosomal DNA to investigate
the phylogeny of Descurainia (Brassicaceae), a genus of perennial woody
shrubs found in the Canary Islands. Low divergence among the cpDNA and
ITS sequences of the seven species supports a recent introduction of the
genus to the islands in a single colonization event. Phylogenetic
reconstruction demonstrates that D. tanacetifolia is the closest
mainland relative to the insular species of Descurainia. This species is
perennial, with subspecies D. tanacetifolia ssp. suffruticosa being
suffrutescent (woody at the base of the stem). The authors conclude that
woodiness was likely present in the continental ancestors of the
Canarian Descurainia, making insular woodiness relictual in Descurainia
(Goodson et al., 2006). They also note that the closest continental
relatives to other Canarian woody, perennial endemics in genera such as
Bencomia (Rosaceae), Convolvulus (Convolvulaceae), and Isoplexis
(Plantaginaceae) are suffrutescent perennials or shrubs.

Other studies have also found relictual woodiness on islands. A
phylogenetic analysis of the fourteen species of Pericallis (Asteraceae)
found in Macaronesia was based on morphological, molecular and combined
data sets (Swenson & Manns, 2003). Pericallis contains both woody
subshrubs and herbaceous perennials. Although not conclusive, the
analysis provided evidence that the ancestral state for Pericallis is
woody, not herbaceous (Swenson & Manns, 2003).

Firm support for relictual insular woodiness has been found for
Lactoris fernandeziana (Lactoridaceae, Juan Fernandez Islands), based on
anatomical, developmental, and molecular data (Fuertes-Aguilar et al.,
2002; Mort et al., 2002). A molecular study using two chloroplast genes
also supports relictual insular woodiness for the Canary Islands species
Plocama pendula (Rubiaceae) (Andersson & Rova, 1999; Bremer, 1996;
Fuertes-Aguilar et al., 2002; Goodson et al., 2006). Tolpis
(Asteraceae), a mostly woody genus from Macaronesia, has also been found
to possess relictual insular woodiness (Mort et al., 2002). In a similar
manner, the molecular phylogeny for all species of Dendrosenecio
(Asteraceae) suggests that its east African highlands ancestors were at
least semi-woody (Knox & Palmer, 1995).

The hypothesis that insular woodiness is derived from mainland,
herbaceous ancestors is supported in a number of taxa. Sideritis
(Lamiaceae) is a genus of rosette plants, suffrutescent perennials, and
arborescent shrubs in Macaronesia, and of suffrutescent annuals and
perennials on the mainland (Barber et al., 2002). Phylogenetic analysis
based on chloroplast and ITS sequences indicates an increase in
woodiness among the insular members, although the ancestral habit was
not firmly identified (Barber et al., 2002). The range of habit on the
islands was interpreted as evidence that the taxa with insular woodiness
are derived from herbaceous ancestors (Barber et al., 2002).

There are several hypotheses that attempt to explain why plants
with secondary insular woodiness have evolved from herbaceous ancestors.
Two of the older hypotheses are Darwin's (1859) competition
hypothesis, and Wallace's (1878) longevity hypothesis.

When Darwin (1839) visited the Galapagos Islands during his famous
voyage on the Beagle, he noticed that the tree sunflowers (Scalesia,
Asteraceae) growing there were woody. He hypothesized, based on their
observed distributions, that while trees were unlikely to reach the
islands (he was unsure why this was), herbaceous plants would be
successful. The herbaceous plants that were successful could gain a
competitive advantage by growing taller. This would lead to selective
pressure for increased woodiness, which would eventually lead to
arborescence (Darwin, 1859; Givnish, 1998). Since seeds or spores of
land plants have to be capable of dispersing over long distances to
reach and colonize islands (Givnish, 1998), the large size and lower
viability of seeds from continental trees may preclude them from
colonizing islands (Carlquist, 1974). The fact that the open, or
partially open, habitats found on islands early in their colonization
lend themselves to colonization by sun adapted herbs also supports
Darwin's hypothesis. Open habitats are likely to be visited by
birds that release seeds via endozoochory (dispersal of seeds from the
digestive tract) or ectozoochory (dispersal of seeds stuck to feathers
or skin) from similar environments on the mainland. As the colonists
become established, there will be a gradual increase in plant coverage,
which will select for the evolution of increased stature that will
likely be accompanied by increased woodiness. Thus, some of the
colonists gain an advantage over other plants in the competition for
light by growing taller (Givnish, 1998). The increase in woodiness that
accompanies the change from open to forest habitats is sometimes
referred to as an ecological shift (Carlquist, 1974). In many cases the
woodier herbs are found in the understory in less open habitats, or
occupy forested areas. They may also grow in scrub habitats (Carlquist,
1974).

Darwin's competition hypothesis has been supported by
simulation models and quantitative measurements in several studies
(Givnish, 1982; Givnish, 1998; Yilman, 1988). In the ALLOCATE simulation
100 species that differed in their allocation to roots, leaves, and
stems competed for a limiting resource and light over ten simulated
years (Tilman, 1988). In nutrient poor soil, species with a high
allocation to leaves (and high relative growth rates) initially
dominated. These species were eventually out competed by species that
had lower allocations to leaves, but greater allocations to roots. None
of the plants which dominated at any point in nutrient poor soil had
high allocations to stems. They were rosette plants. In three other
scenarios, each having progressively richer soils, species with a high
allocation to leaves and low allocation to stems initially dominated.
However, over time these species were replaced by plants with higher
allocations to stems and roots. The dominant species at the end of the
ten year simulation period had a greater allocation to stems than leaves
in all three of the nutrient rich plots (Tilman, 1988). This suggests
that woody plants will be favored under these conditions.

Field studies and observations also support the competition model.
A study of 72 herbaceous species found along a gradient from a dry oak
to a floodplain forest used data on leaf coverage (the average density
of foliage within a [m.sup.2] plot) and maximum leaf height (plant size)
to investigate the adaptive significance of height in herbaceous plants
in forests (Givnish, 1982). Each species was placed into a maximum leaf
height category, and leaf coverage for each was estimated at 13
intervals for each plot. Maximum leaf height was correlated with the
average density of leaf coverage in the habitat. With each 7% increase
in herbaceous leaf cover, maximum leaf height roughly doubled (Givnish,
1982). The correlation between plant height and percent cover confirms a
prediction of Darwin's hypothesis.

A similar relationship between leaf height and density of leaf
coverage was found in a field study of orchids in the northeastern
United States. Taller species like Cypripedium reginae, Platanthera
(Habenaria) ciliaris, and P. (Habenaria) dilatata were found in habitats
such as meadows, bogs, and swamps where there are dense layers of
herbaceous competitors. Species of intermediate height like Cypripedium
candidum, and members of the genera Calopogon and Arethusa, occurred in
less dense communities. The shortest orchids were found in habitats with
low densities of competitors, heavy shade, or both. For example, the
basal leaved Habenaria straminea grew in open sites with little
competing vegetation, while the caulescent Cypripedium arietinum was
found in shady woods with low densities of competitors (Givnish, 1982).

In contrast to Darwin's competition hypothesis, Wallace (1878)
hypothesized that woodiness would evolve in herbaceous island colonists
as a way to extend their life span. With greater longevity would come
more flowers, which would allow a greater chance for cross-pollination
by insect pollinators, which were then thought to be less common on
islands (Carlquist, 1974; Givnish, 1998; Jorgensen & Olesen, 2001;
Wallace, 1878). Unfortunately, this hypothesis has not been well
supported. First, many plants can self-pollinate, obviating the need for
insect pollination. Also, it turns out that insect pollinators are not
uniformly rare on islands. They tend to be either very abundant (if they
have no competitors), or else they are entirely absent if they have not
dispersed to the island (Carlquist, 1974; Givnish, 1998).

Some support for Wallace's (1878) longevity hypothesis has
come from a study of Echium, a genus of mostly woody perennials from
Macaronesia. All but two of the island species (E. bonnetii and E.
pitardii) are woody, while the continental species of Echium are
predominately herbaceous (Bohle et al., 1996). The study used noncoding
DNA from the chloroplast ([trnT.sub.UGU]-[trnL.sub.UAA] spacer,
[trnL.sub.UAA] intron, [trnL.sub.UAA]-[trnF.sub.GAA] spacer) and nuclear
genomes (18S-5.8S ITS-I region). The DNA was isolated, amplified and
sequenced, and the sequences were aligned visually. Phylogenetic trees
were constructed using maximum parsimony and neighbor joining methods.
The resulting phylogeny supports the idea that the island species are
derived from continental ancestors. In addition, two length
polymorphisms (indels) in common between the island and the continental
species provided support for a single, mainland-to-island invasion, with
rapid speciation by the colonizers (Bohle et al., 1996).

The woody habit of insular Echium has been hypothesized to be an
adaption that helped prevent inbreeding depression in the geographically
isolated founding populations (Bohle et al., 1996). Echium is insect
pollinated. Since the islands of Macaronesia are insect poor, it is
unlikely that its initial environment on the islands would have
contained many insects. Under these conditions, increased woodiness
would extend the life of the plant and would be expected to provide an
opportunity for increased pollination. Since the mainland, herbaceous
members of Echium show inbreeding depression, it is likely that the
colonizing species would also have had this characteristic. This would
have lead to selection for outbreeding among the founding members of the
species. Selection for outbreeding could be expected to lead to the
large inflorescences found in the current island species. These large
inflorescences could help attract pollinators, while the woody habit
would provide the structural support necessary to produce the
inflorescences.

A third hypothesis for the evolution of woodiness in insular plants
was stimulated by the observation that islands have moderate climates
(Carlquist, 1974; Jorgensen & Olesen, 2001). Island climates are
either uniform throughout the year, or else have their climatic extremes
moderated by oceanic influences. Climatic moderation includes moderate
annual temperatures, adequate and uniform rainfall, and high humidity.
Moderate annual temperatures means temperature ranges of 10-25[degrees]C
(Hawaii, Fiji, New Caledonia), or 5-20[degrees]C (Juan Fernandez, St.
Helena). Annual rainfall of at least 1,000 mm is found on islands such
as Juan Fernandez and St. Helena, at least at sea level. In association
with rainfall, high humidity resulting from the nearby ocean can provide
insulation against temperature fluctuations. High humidity also helps
reduce transpiration, as well as evaporation from the soil. Humidity in
some locations in Hawaii averages 81%, while in the Canary Islands 75%
humidity is not uncommon (Carlquist, 1974). Moderation in these
components provides a release from seasonality for the island plants, so
that continuous growth is possible. Continuous growth allows a root
system that can support more leaf growth and greater arborescence
(woodiness), which permits additional photosynthesis (Carlquist, 1974).

Some small evidence against this hypothesis comes from the Echium
study (Bohle et al., 1996). In moderate climates comparable to that of
Macaronesia, such as the Iberian Peninsula, this hypothesis predicts
that Echium should be woody, as it is in Macaronesia. However, all
species of Echium on the Iberian Peninsula are herbaceous.

The final hypothesis for the evolution of insular woodiness is
related to the absence of herbivores on islands. The presence of large
herbivores tends to select for rapid completion of plant life cycles in
order to avoid predation. Release from this selective pressure allows
plants to complete their life cycles, and to exploit climatic
moderation. Thus, rather than being consumed, the plants enjoy
year-round growth, which eventually favors the evolution of increased
woodiness and perennial growth forms (Carlquist, 1974; Jorgensen &
Olesen, 2001). Supporting this idea is the fact that on islands where
herbivores have been introduced predatory pressure has prevented the
reproduction of some woody plants. On these islands, only annuals and
woody plants on inaccessible cliffs tend to survive. On other islands
such as Hawaii, the effect of long-term herbivory is drastic enough to
threaten the survival of some woody, and woody herbaceous species
(Carlquist, 1974).

D. Secondary Woodiness

Like paedomorphosis and insular woodiness, the term secondary
woodiness is not always used consistently in the literature. Secondary
woodiness is best described as the evolution of secondary xylem in
plants with herbaceous ancestors (Carlquist, 1974, 1992; Lens et al.,
2005a). The term itself suggests that woodiness was lost, and then
reappeared during later evolution (Isnard et al., 2003). It also implies
that primary woodiness, a term rarely used in the literature, exists.
Primary woodiness may be inferred to mean woodiness derived from a woody
ancestor (i.e., plesiomorphic woodiness) (Carlquist, 1995b; Carlquist,
2003).

As has been made clear above, neither the existence of
paedomorphosis nor insular woodiness means that secondary woodiness must
necessarily be present. It is true that many plants with paedomorphic
wood are secondarily woody, and that insular woodiness is frequently
associated with secondary woodiness, but the correlations between these
characteristics are not absolute. The mere existence of paedomorphic
characteristics is insufficient to determine if a plant has secondary
woodiness. Likewise, the existence of insular woodiness is not
incontrovertible evidence of secondary woodiness (Kiln et al., 1996). In
some cases, insular woodiness has been found to be plesiomorphic
(relictual). Nor can insular woodiness be equated with paedomorphosis.
Plants with insular woodiness may or may not have paedomorphic wood.
Phylogenetic analysis must be used to resolve these questions. We return
to these points in the Discussion.

In order to investigate the relationship between paedomorphosis and
secondary woodiness in a specific context, three potentially secondary
woody species were chosen for study. Carlquist (1995b) reports the
presence of paedomorphic rays in Xanthorhiza simplicissima, and suggests
that this species may be secondarily woody. However, he recommends study
of larger, older stems to confirm the ray type (Carlquist, 1995b). Both
Carlquist (1974, 1985) and Thorne (1969) consider Coreopsis gigantea to
be an example of insular woodiness, but neither addresses the issue of
secondary woodiness. Carlquist (1995b) suggested that Mahonia bealei
might be secondarily woody based on interpretations of the position of
the genus in published cladograms (Loconte & Estes, 1989; Qiu et
al., 1993). Loconte and Estes (1989) hypothesize a woody ancestor for
Berberidaceae, with a shift to herbaceousness, and then a shift back to
woodiness for the branch bearing the genera Berberis and Mahonia.
Investigation of these three species provides the opportunity to discuss
the concepts of paedomorphosis, insular and secondary woodiness in a
specific context.

B. Organography and Anatomy

1. Xanthorhiza simplicissima

Xanthorhiza simplicissima (yellowroot) is a monotypic, perennial,
small deciduous shrub. Its stems rarely reach more than a meter in
height, and 3-10 mm in diameter. Younger stems are relatively erect,
while older stems may become partly procumbent (Fig. 2a). Very young
stems have a ring of lignified primary fiber bundles similar to those
found in species of Clematis (Isnard et al., 2003). Older stems have
dense wood with small groups of relatively narrow diameter vessels, and
conspicuous large unlignified rays. The dense wood found in X.
simplicissima is rare in the Ranunculaceae (Isnard et al., 2003; Rowe et
al., 2004).

The leaves of X. simplicissima are alternate, pinnately or
bipinnately compound, are usually divided into three to five leaflets
that are serrate to deeply toothed, and have long slender petioles (Fig.
2a). The bark is gray-brown and smooth, with the inner bark
yellow-colored due to the presence of berberine. The flowers are yellow
to purple-brown, have five petals, and appear in early spring.
Individuals are usually found in shady, damp woods near water, from New
England to northern Florida. Yellowroot was widely used for medicinal
purposes by Amerindians. It has also been used in folk medicine in the
American South (Reed, 2004; Seiler et al., 2008).

Many Ranunculaceae are reported to be annuals with short-lived
stems and only primary growth, thought recent studies have begun to
question these descriptions (Table 1) (Schweingruber & Landolt,
2005-2008). Clematis and Xanthorhiza, at least, are exceptions to these
rules for they have distinctly woody stems. However, in neither case are
the stems strongly self-supporting. Growth forms in these genera include
lianas, creepers, and small shrubs which are either partially
self-supporting or procumbent (Isnard et al., 2003; Rowe et al., 2004).

2. Coreopsis gigantea

Although most Asteraceae are herbaceous, some family members have
structural forms that include herbs, shrubs, and trees (Cronquist, 1955;
Heywood et al., 1977; Metcalfe & Chalk, 1950). Of these forms, the
herbs and shrubs tend to be found in regions where there is the greatest
phyletie diversity of other taxa. This is frequently the xeric montane
areas of subtropical and tropical North and South America. In contrast,
trees or tree-like forms tend to be found in areas of low phyletic
diversity, such as middle and low tropical or subtropical rain forests.
They also occur as insular endemics (Heywood et al., 1977).

[FIGURE 2 OMITTED]

Coreopsis gigantea, giant Coreopsis, is a perennial shrub ranging
from 0.45 m to 1.2 m tall (Fig. 2b). It occurs on rocky ocean cliffs and
dunes of the coast sage shrub and coastal strand plant communities of
California. It is drought tolerant, grows best under full sun, and lives
in well-drained soil. The leaves are fern-like and pinnately divided
into linear segments clustered at the ends of the branches. It blooms
March to May with flowers that resemble the common daisy. The heads
occur in cymes on long scapiform peduncles. Although Carlquist (1985)
classified C. gigantea as an herb due to its degree of succulence, it
does not fit well into this, or any, common category of growth forms.
The trunk of C. gigantea is sturdy, though somewhat more pliable than a
small tree. It usually has a cluster of small branches at the top (Fig.
2b) (Keil, 1993; USDA & NRCS, 2009).

3. Mahonia bealei

The Berberidaceae consists of both herbs and shrubs from mostly
Northern temperate regions, with some species from the Andes. Species
occur in habitats ranging from forest understories, to arid regions and
deserts. The family often contains alkaloids such as berberine, an
isoquinoline that colors the wood yellow (Judd et al., 2002; Metcalfe
& Chalk, 1950). Phylogenetic study places the family as the sister
group to the Ranunculaceae (Hoot et al., 1999). The 70 species of
Mahon& are evergreen shrubs native to eastern Asia, North and
Central America. Several species are grown ornamentally.

Mahonia bealei is a typical example of the cultivated members of
the genus. It is a 0.5-4(-8) meters tall evergreen, perennial shrub
native to eastern Asia (Fig. 2c). In China it occurs in forests, forest
margins, weedy slopes, streamsides, roadsides, and thickets at 500-2000
m (Ying et al., 2010). Its leaves are alternate and pinnately compound,
and composed of nine to fifteen opposite, ovate leaflets that have spiny
dentate margins. The yellow flowers emerge in spring and, in late
summer, produce blue-black berries covered in a waxy white bloom. It
grows in a rounded, open, irregular habit with upright branching (Kling
et al., 2008).

C. Materials and Methods

Specimens of Xanthorhiza simplicissima Marsh. (Fig. 2a) were
collected from three sites in North Carolina: Guilford County (the
Randleman Lake area), Randolph County (Little River at Pisgah Covered
Bridge; voucher: NCU 589175, 589174), and Transylvania County (the Avery
Creek area of Pisgah National Forest). Coreopsis gigantea (Kellog) H. M.
Hall (Fig. 2b) was collected from the University of California at
Berkeley Botanical Garden (accession number: 60.0129). Mahonia bealei
(Fortune) Carr. (Fig. 2c) was collected in Guilford County, North
Carolina on the campus of the University of North Carolina at Greensboro
(voucher: NCU 589177, 578176), and at 309 Waverly Way, Greensboro, NC.

All plant material was stripped of leaves, and the stems cut into
short sections of approximately 2.5 cm length. The sections were then
fixed in either FAA (formalin-acetic acid-alcohol), or Karnovsky's
fixative (Berlyn & Miksche, 1976; Ruzin, 1999).

Following fixation, slides for microscopic study were made from
either stem sections, or macerated tissue. Stems with diameters of 3.5,
4.0, 4.5, 5.0, and 6.0 mm were used for measurements and observations of
Xanthorhiza simplicissima, with the larger diameters (4.5, 5.0, 6.0 mm)
utilized predominately. A small stem with a diameter of 19.0-23.0 mm,
and a larger stem with a diameter of 52.0-58.0 mm were used from
Coreopsis gigantea. Mostly larger stems (8.0, 8.5, 9.5 mm diameters)
were sectioned for M. bealei, but some 6.0 mm diameter stems were also
used. Stem sections (transverse, radial, and tangential) were made
either by hand using a razor blade, or with an American Optical Model
860 sliding microtome.

After sectioning, all sections were transferred to 50% ethyl
alcohol. The sections were then stained with 1% saffanin in 95% ethyl
alcohol, which was sometimes followed with fast green (Ruzin, 1999).
Following staining, the sections were either mounted directly, or washed
with 95% and 100% ethyl alcohols, and finally Clear Rite II
(Richard-Allan Scientific, Kalamazoo, MI), in place of Xylene. The
sections were placed on glass slides and covered with either
Richard-Allan Slide Preparation Mounting Medium (Richard-Allan
Scientific, Kalamazoo, MI) or, for sections mounted from aqueous media,
Karo Light Corn Syrup (ACH Food Companies, Inc., Memphis, TN), followed
by cover slips.

For observation with the confocal microscope specimens fixed in FAA
were sectioned by hand using a razor blade to prepare transverse,
tangential, or radial sections. The sections were stained with 1%
saffanin in 50% ethyl alcohol for twenty to thirty minutes, and then
dehydrated through a graded acetone series at 15-30 minute intervals, at
concentrations of 30%, 50%, 75%, 90%, and 100% acetone. The sections
were then rehydrated through a graded acetone series (90%, 75%, 50%,
30%, 10% acetone) to distilled water using the same time intervals
(Kitin et al., 2002; Kitin et al., 2000). Finally, the sections were
cleared in a graded glycerol series (25%, 50%, 75%, 100% glycerol).
Specimens were kept in each glycerol solution for at least one hour. The
cleared sections were mounted on glass slides using 100% glycerol, cover
slipped, and photographed with an Olympus FluoView[TM] FV500 confocal
laser scanning microscope (Kitin et al., 2000).

Macerations were prepared following Gifford's method after
first removing the periderm from the stems (Ruzin, 1999). Once
maceration was complete, the tissue samples were washed with distilled
water and transferred to 50% ethyl alcohol. Samples were placed on glass
slides, teased apart, and stained for ten minutes with 1% safranin in
95% ethyl alcohol. The stained tissue was washed in several changes of
95% followed by 100% ethyl alcohol. A final wash with Clear Rite was
completed before the macerations were placed on slides and covered with
Richard-Allan Mounting Medium (Ruzin, 1999).

Olympus CH-2 and Leitz Ortholux II light microscopes were used to
take measurements of cells and tissues, as well as for general tissue
observation. Photographs were taken on a Nikon Eclipse E600 light
microscope equipped with a Diagnostic Instruments, Inc. Model 2.2.1
Revision 2 digital camera, and SPOT Advanced Software 4.0.8, and on a
Leitz Ortholux II light microscope equipped with a Leica DFC420 digital
camera and Image-Pro Express 6.0 software. Field photographs were taken
with a Nikon CoolPix 5700 model digital camera. The tonal qualities of
the images were adjusted, labels were added, and plates assembled with
Adobe Photoshop CS, CS3 and CS4. Figure 1 was redrawn from Carlquist
(1962) in Adobe Illustrator CS.

Cell dimensions were measured at the longest and widest points of
the cells. For vessel elements, measurements included the cell walls and
caudate tips. Measurements for vessel elements and fibers came from
macerations. Trends of vessel element lengths across the secondary xylem
were determined from measurements taken from radial sections. Vessel
element lengths were measured across the width of the xylem, from xylem
formed at the edge of the pith to that adjacent to the vascular cambium.

Widths of rays were measured in tangential sections, while ray cell
dimensions (height and width) were measured in radial sections.
Tangential sections were used to determine if vessel elements exhibited
storying (adjacent elements with end walls aligned with each other).
Radial sections also provided insight into the extent of vessel, ray,
and fiber storying. The mean number of vessels per group, vessel
restriction patterns, and vessel distribution were determined from
transverse sections.

Statistical analyses were carried out with Minitab 15 statistical
software (Minitab Inc., 2006), and SPSS (SPSS, 2006). Scatterplots with
least squares regression lines of vessel element lengths from across the
width of the xylem were constructed in SPSS and refined in Adobe
Illustrator CS3. LOWESS (Locally Weighted Scatterplot Smoother) curves
were also plotted in SPSS. LOWESS is a robust method of smoothing
scatterplots by using locally weighted regression, which minimizes the
influence of outliers on the smoothed lines (Cleveland, 1979, 1981).

A word of caution about the statistical analysis is appropriate.
Since only a few plant samples were taken from a limited number of
sites, it is not statistically valid to make inferences about the
species populations based on these results. To ensure statistically
robust inferences about the population, an appropriate random sampling
design would be necessary (Ramsey & Schafer, 2002). While most plant
anatomical studies fall short in this area, means are still calculated
and used in traditional plant studies. In light of this fact, the means
and other measurements provided here may still provide useful insights.

To determine if the study species are secondarily woody, the
character states "woody" and "nonwoody" were plotted
on recent phylogenies containing the study taxa. The tree of Kim et al.
(2004a) was used to determine the origin of woodiness in Xanthorhiza.
For Coreopsis we used Mort et al.'s (2004) phylogeny of the western
clade of Coreopsis (Archibald et al., 2005; Mort et al., 2004). Kim et
al.'s (2004a) phylogeny of the Berberidaceae was the basis for
determining if secondary woodiness is present in Mahonia.

In order to determine if the other taxa on these phylogenetic trees
are woody, a character matrix was created based on the published
descriptions of each taxon. Information such as habit, longevity, and
morphological descriptions of the taxa were used to judge the likelihood
of woodiness in the taxa (Tables 2, 3, and 4). Except for in Coreopsis,
determinations of woodiness were based on our subjective probability of
the presence of a significant amount of fibrous wood in each species.
Use of this rather strict definition of wood is necessary because recent
work has shown that many taxa previously considered to lack secondary
growth actually possess it (Dietz & Ullmann, 1997; Dietz &
Schweingruber, 2002; Krumbiegel & Kastner, 1993; Schweingruber,
2006; Schweingruber, 2007a; Schweingruber & Poschlod, 2005;
Schweingruber & Landolt, 2005-2008). We return to this issue in the
Discussion. Once each of the taxa was assigned a character state (woody
or nonwoody), Mesquite 2.0 was used to plot woodiness on the published
phylogenetic trees of the taxa (Maddison & Maddison, 2009).

To investigate the occurrence of paedomorphic wood in primitively
woody taxa we plotted data on the co-occurrence of scalariform
perforation plates and uniseriate homocellular rays with upright cells
(Paedomorphic Ray Type III) on a phylogeny of the Ericaceae (Kron et
al., 2002). The data for these plots was extracted from the Xylem
Database (Schweingruber & Landolt, 2005-2008). We choose the
Ericaceae for analysis because Meylan and Butterfield (1978) report
several species with these paedomorphic characteristics.

a. Overview. Transverse sections of Xanthorhiza simplicissima
reveal a pith composed of large thin-walled, isodiametric parenchyma
cells. In radial and tangential views, these cells appear somewhat oval
with horizontal sides that are often parallel, and flattened (Fig. 2d).
At the pith/xylem interface, the parenchyma cells of the pith tend to
have an upright (vertical) orientation.

Close to the xylem, the cells are smaller in transverse section
and, at the base of the earliest formed vessel groups, they transition
to much smaller, radially elongated, oval shaped cells (Fig. 2e). These
cells gradually assume somewhat rectangular shapes between the strands
of vessels, which radiate outward from the earliest formed clusters of
vessels. Proceeding radially outwards, the parenchyma cell strands
(primary rays) become the rays of the secondary xylem. No rays form
within the primary vascular bundles, or their radial extensions. All of
the vessels lie between the rays, and are embedded in fiber cells. The
fiber cells are smaller than the ray cells, and are oval or rounded in
outline, as seen in cross-section (Fig. 2e, f).

The xylem occupies a major proportion of the stem, especially in
older stems, and forms a cylinder around the pith (Fig. 2g). The
secondary xylem is composed only of vessels, rays, and fibers, with few
or no tracheids. No axial parenchyma or fiber tracheids were found,
Curved arches of tissue composed of parenchyma cells interspersed with
half moons of partially crushed phloem occur just outside the secondary
xylem and vascular cambium. The phloem is frequently capped with small
arches of fibers. Beyond the fibers lies the periderm (Fig. 2h).

b. Vessels. Vessels form a prominent feature of the secondary
xylem. In transverse section they appear rounded or oval, and may have
flattened sides, especially where they come into contact with other
vessels. The vessels occur in single, or occasionally double, radial
strands that radiate outwards from the beginning of a growth ring (Figs.
2f-h). They are distributed in a semi-ring porous pattern. A typical
growth ring is marked by a short tangential band of wider diameter
vessels at the beginning of the ring, followed by a discontinuous strand
of smaller vessels that ends at the margin of the next growth ring (Fig.
2i). In many transverse sections, a darker stained line of cells can be
seen at the end of a growth ring. This is due to the slightly smaller
cell size of the fibers and ray cells produced at this position (Fig.
2j). Almost all of the vessels are restricted to the central part of the
area between the rays, with no contact with the parenchyma cells of the
rays (Fig. 2i).

There are a substantial number of single vessels that are separated
from one another by intervening fiber cells. There are also a
substantial number of pore multiples, vessel groups containing two,
three, or more vessels (Fig. 2e). Extremely high numbers of vessels
making contact in a group are not common, but there are examples of
groups with ten, sixteen, or even more vessels. The mean number of
vessels in a group is 2.5 (n=26; SE=0.1).

The mean length of the vessel elements is 249.4 [micro]m (n=50;
SE=9.4), and the mean width is 32.9 [micro]m (n=50; SE= 1.6). Narrow,
oval shaped pits in alternating rows are found on the secondary walls of
the vessel elements. The pits are numerous, lack borders, and cover much
of the lateral surface of the vessels. No examples of vessel elements
with annular rings, helices, scalariform or reticulate secondary wall
deposition patterns were found. Therefore, the only part of the surface
of the primary wall that is not covered by the secondary wall is the
part exposed due to the pit apertures (openings) (Fig. 2k).

The vessel elements have simple perforation plates set in end walls
that are slightly to fairly oblique (Fig. 2k), with noticeable caudate
tips, which can be relatively long (Fig. 21).

There is no overall vessel element storying, though it is
occasionally possible to find elements in adjacent vessels that are
aligned with each other (Fig. 2m).

A graph of vessel element lengths across the xylem with a Locally
Weighted Scatterplot Smoother line fitted shows vessel element length
decreasing with distance from the pith (Fig. 3b). More recently formed
vessel elements have shorter lengths than the older vessel elements. The
resulting curve has an overall negative slope (Fig. 3b). When a linear
regression line is fitted to the same data points, a negatively sloped
line also results (p=0.00, [r.sup.2]=0.104; Fig. 3a).

c. Rays. Rays in Xanthorhiza simplicissima are multiseriate, and
are composed of upright (or square) ray cells (Paedomorphic Type II). In
transverse sections, they originate as two to four cell wide primary
rays in the earliest formed (primary) xylem, and gradually widen as they
extend across the secondary xylem towards the vascular cambium. The rays
do not branch, and no rays originate within the vascular bundles (Fig.
2e-h). The ray cells tend to be somewhat rectangular in outline in
transverse views (Fig. 2e, j). In tangential view they may be rounded,
but most are oval or elongated oval in shape (Fig. 2m). Some of the
elongated ray cells have flattened or angled ends (Fig. 2n). Many are
densely packed with inclusions that may be starch grains (Fig. 2n). The
lateral cell walls of the cells have scattered, oval-shaped pits, which
may occur in an alternate pattern, when they occur frequently.

The rays tend to be tall, and always extended past the end of the
tangential sections. Due to this, maximum ray height could not be
measured. They are at least 6 mm high, the size of the largest sections.
The rays have a mean width of 150.3 [micro]m (n=26; SE= 12.4), or 9.6
cells (n=26; SE=0.8). In tangential section most of the rays have
uniseriate ends, some terminating in a single cell. However, it is also
common to find rays where the uniseriate ends extend for four or five
cells. None of the rays are storied (Fig. 2n). All ray cells are upright
when seen in radial section. Mean ray cell height is 55.1 [micro]m
(n=54; SE=2.2), and mean width is 22.4 [micro]m (n=54; SE=0.9).

[FIGURE 3 OMITTED]

d. Fibers. Only libriform fibers are found in the secondary xylem.
This fiber type characteristically has tapered ends, although forked
ends can be found occasionally. In transverse section the fibers are
rounded or somewhat oval, with small lumens (Fig. 2e, i, j). The fibers
have pit apertures that have no apparent borders, and resemble slits
(simple pits). Their mean length is 351.1 [micro]m (n=50; SE=8.8), and
mean width (including the cell wall) is 15.9 [micro]m (n=50; SE=0.5).

Libriform fibers are very abundant throughout the secondary xylem,
forming a matrix in which the vessels and rays are embedded (Fig. 2h).
Some of the fibers close to ray cells appear to contain starch grains
(Fig. 2n). Fiber cells farther away from the rays do not. No storying
occurs in the libriform fibers (Fig. 2m, n).

e. Secondary Woodiness. Based on our character state determination
(Table 2), and the phylogenetic tree of Kim et al. (2004a), the
ancestral state of X. simplicissima is equivocal (Fig. 4). The outgroup,
Euptelea, is a deciduous tree (Kim et al., 2004a).

[FIGURE 4 OMITTED]

2. Stem Anatomy and Xylem Characteristics of Coreopsis gigantea

a. Overview. Transverse sections show a stem and xylem anatomy
quite different from Xanthorhiza simplicissima. In general, ground
tissue made up of parenchyma cells and containing vascular bundles of
xylem and phloem arranged in a ring dominates Coreopsis gigantea stems
(Fig. 5a). While some observations were based on young stems,
measurements for assessing paedomorphosis were based on larger stems
because they have more substantial amounts of secondary xylem. There are
also developmental differences between the stem and xylem anatomy of
young (smaller) and older (larger) stems.

b. Parenchyma in Younger Stems. Parenchyma cells throughout younger
stems are thin walled. In the center of the stem, in transverse section,
they are rounded, or slightly oval in outline, and frequently have
flattened sides where they make contact with adjacent parenchyma cells.
In general, the cells are smaller toward the vascular bundle region, and
then become larger again moving towards the outside of the stem, and in
the cortex. However, parenchyma cells found in the outer part of the
stem tend to be smaller than those located in the center of the pith.
Within the pith, the cells are much smaller near the earliest formed
vessels of the vascular bundles than in the center. In some cases, they
retain their rounded shape; in others they are more oval, or even
elongated, near the vessels. A ring of secretory canals occurs in the
pith, close to the earliest formed vessels (Fig. 5b).

[FIGURE 5 OMITTED]

The rays are all multiseriate, with no uniseriate rays
(Paedomorphic Type II). The majority of the ray parenchyma is produced
by the action of the vascular cambium during secondary growth (Fig. 5c),
and occurs between the smaller fascicular regions. Some of the smaller
ray cells are shaped like elongated ovals, while others are only
slightly oval shaped. Elongated, oval shaped parenchyma cells are most
common where the rays are narrow. In very narrow rays, elongated
parenchyma cells with somewhat flattened ends, tending towards a
rectangular shape, are also seen. These rectangular parenchyma cells are
also present between phloem strands, as a continuation of the xylary
rays. If the rays are wide, the constituent parenchyma cells are more
rounded.

Just outside the most recently formed vessels lie strands of phloem
embedded in parenchyma (Fig. 5d). A second ring of secretory canals lies
outside the outer boundary of the phloem (Fig. 5c). The parenchyma cells
located in this region have more rounded outlines, becoming oval-shaped
outside the secondary phloem, near the third ring of secretory canals in
the outer cortex, just inside the periderm (Fig. 5b, c). These outermost
parenchyma cells have an upright orientation when seen in radial or
tangential section.

c. Vessels in Younger Stems. Vessels typically occur in short
wedge-shaped or roughly rectangular groups, as seen in transverse
section. Some vascular bundles lie close to each other, with narrow
interfascicular regions. Others are separated by much wider
interfascicular regions (Fig. 5c). This difference probably reflects the
position of the section in relation to anastomoses between vascular
strands. The vessels themselves are usually arranged in multiple,
discontinuous, radially oriented strands (Fig. 5b). Vessels found on the
borders of the bundles often make contact with the parenchyma of the
interfascicular regions. Beyond this, however, it is difficult to
determine a distinct pattern of distribution of the vessels in the
vascular bundles. For example, there may or may not be gaps in the
vessel strands, and obvious trends in the diameter of the vessels are
hard to discern. At least in some cases, there may be gaps in the vessel
strands at the same positions in adjacent bundles. These gaps likely
signal the transition from one growth increment to the next.

d. Parenchyma in Older Stems. Many of the observations made on the
transverse sections of younger stems hold true for the older stems, but
there are some differences. There is a tendency for the parenchyma cells
in the middle of wide rays to be noticeably larger than those lying
close to the vessels. The cells tend to be round at the stem center,
elongated-oval shaped between the vascular strands, and are somewhat
rectangular adjacent to the most recently formed vessels. In the
vicinity of the youngest vessels many of the ray parenchyma cells are
rectangular, or elongated ovals with flattened ends. The rays tend to be
more consistent in width when compared to younger stems, although there
are exceptions.

e. Vessels in Older Stems. Vessels in larger stems are distributed
in a semi-ring porous pattern. Strands of vessels typically extend
radially from a cluster of vessels at the beginning of a growth
increment. Vessels located on the borders of vessel groups often make
contact with the parenchyma cells in the adjacent rays.

The vessel clusters, which consist of larger diameter vessels
(sometimes only marginally larger), have a tangential or sometimes a
diagonal orientation in transverse sections, and are not always well
defined. They may consist of only one or two large diameter vessels
(Fig. 5e). These conglomerations of vessels represent the earlywood
(i.e. springwood). The vessel strands that extend from these clusters
form the latewood (i.e. summerwood, Fig. 5f). The vessel strands are
discontinuous in that they often contain single vessels that do not make
contact with each other, as well as groups of two or three vessels in
contact with each other. Mean group size is 1.7 vessels (n =26; SE=0.1),
although the range extends to 11 vessels per group.

[FIGURE 6 OMITTED]

There is often a noticeable space at the end of the latewood where
the vessel strands end in parenchyma, or in a few fibers (Fig. 5f, g).
Another tangential cluster of vessels occurs radially outside this area
(Fig. 5f), beginning a new growth increment. Thus, in older stems the
vessels exhibit a repeating pattern of a tangential vessel cluster, a
radiating strand of vessels, and a band of parenchyma. The cambial zone
can be seen as a darker staining ring where the vessel strands terminate
(Fig. 5e).

The vessel elements have a mean length of 180.2 [micro]m (n=50;
SE=5.9), and a mean width of 51.6 [micro]m (n=50; SE= 1.8), as
determined from macerations. They frequently widen in circumference at
theft ends, and occasionally have slight caudate tips (Fig. 5h). Most
often they have fairly transverse or perhaps slightly oblique end walls.
A few of the vessel elements have helical lateral wall deposition
patterns, but the majority have helical transitional to scalariform, or
pseudo-scalariform deposition patterns (Fig. 5i). Scalariform lateral
wall patterns are the most common. No lateral wall pits are visible on
the vessel elements (Fig. 5h, i). The end walls have simple perforation
plates. The vessels tend to show storying in tangential views (Fig. 5j).

Graphs of vessel element lengths measured across the width of the
xylem from a 22 mm stem show a decrease in vessel element length, with
more recently formed vessel elements having shorter lengths than the
earlier formed elements (Fig. 6). The resulting LOWESS curve initially
declines, then levels out (Fig. 6b). The overall slope is negative. When
a linear regression line is fitted to the data, a negatively sloped line
results (p=0.00, [r.sup.2]=0.101; Fig. 6a).

A second set of vessel lengths from a stem with a 58.0 mm diameter
also produced a consistently negatively sloped curve, as did a linear
regression line fitted to this data (p=0.00, [r.sup.2]=0.175; Fig. 7). A
third set of data points taken from the same 58.0 mm stem produced a
nearly flat curve, and a linear regression line with essentially no
slope (p=0.67, [r.sup.2]=0.001; Fig. 8).

[FIGURE 7 OMITTED]

f. Rays in Older Stems. Like their younger counterparts, older
stems have multiseriate, non-storied rays (Paedomorphic Type II). In
radial sections the shapes of the parenchyma cells follow the same
general trends as seen in transverse sections. The stem center has
larger, rounded parenchyma cells, while the cells around the earliest
formed vessels are oval or sometimes a little elongated, and tend to
have an upright orientation. Moving radially outward rounded parenchyma
cells still occur between the vessel strands, but vertically oriented
cells increase in number and become more common, especially immediately
adjacent to vessels. New rays may form amid the vessels, splitting the
radial files of cells (Fig. 5g, arrowhead).

Tangential sections of older stems show the same general tendencies
in parenchyma cell shape as in younger stems, but rounded parenchyma
cells are much less common. Elongated, oval parenchyma cells with
vertical orientations predominate (Fig. 5k). The parenchyma cells
directly adjacent to vessels, and in very narrow rays between strands of
vessels, are typically narrower than in other locations. The cells in
these areas are also more elongated and have angled, slightly pointed or
flattened ends. Some of the cells are coffin shaped (Fig. 5k).

[FIGURE 8 OMITTED]

The height of the rays in tangential section could not be
determined because they exceeded the height of the sections (5.67 mm),
Mean width of the most recently formed rays is 134.5 [micro]m (n=20;
SE=17.5), or 5.6 cells (n=20; SE=0.50) (Fig. 5k).

Radial sections reveal ray parenchyma cells that are rectangular in
outline. Of these, most are upright, with a few squarish cells
intermixed. Only a few procumbent cells are found. Measured in radial
section, the mean height of the cells is 113.2 [micro]m (n=50; SE=3.7),
and the mean ray cell width is 69.5 [micro]m (n=50; SE=2.4) (Fig. 51).

g. Fibers in Older Stems. Libriform fibers are not common, but do
occur in the xylem of Coreopsis gigantea. In transverse section the
fibers appear rounded or oval in outline. Macerations show that most are
tapered, although examples of fibers with forked ends can occasionally
be found. The fibers have a mean length of 336.7 [micro]m (n=53;
SE=10.7), and a mean width of 27.1 [micro]m (n=53; SE=1.1).

When they occur: libriform fibers are closely associated with
vessels, and normally are found next to single vessels or groups of
vessels. If they are not immediately adjacent to a vessel, they are
separated by only a few parenchyma cells. The fibers never occur as
isolated cells in areas of parenchyma far from the vessels. Wherever
they are located, they usually occur in either strands of single fibers,
or strands that are 2-4 fibers wide. A few fiber strands are
considerably wider, 7-8 fibers in width. Many of the multiseriate fiber
strands are storied, at least to some degree.

Other than vessels, libriform fibers, and parenchyma cells, no
other cell types occur in the xylem. There is no evidence for tracheids
or fiber-tracheids.

h. Secondary Woodiness. Plotting woodiness on a phylogenetic tree
of the Coreopsis species from western United States, Mexico, and Central
America, supports the secondary derivation of woodiness in C. gigantea
(Fig. 9).

3. Stem Anatomy and Xylem Characteristics of Mahonia bealei

a. Overview. The stem anatomy of Mahonia bealei resembles that of
Xanthorhiza simplicissima. Like Xanthorhiza, transverse sections of M.
bealei show a prominent pith composed of parenchyma cells, surrounded by
a cylinder of xylem (Fig. 10a). Just beyond the vascular cambium are
arches of compressed phloem mixed with parenchyma, which form convex
protrusions capped by phloem fibers. The parenchyma cells located to the
outside of the phloem are larger than those in the rays. Irregular
shaped fiber bundles are scattered among the parenchyma cells of the
cortex (Fig. 10a).

In the main part of the pith, parenchyma cells tend to be fairly
large and round (Fig. 10b). In radial and tangential sections the
parenchyma cells are in vertical files, are upright, and are rectangular
or squarish (Fig. 10c). Their end walls are often angled, and their side
walls may be curved, or at least not parallel, so that the files of
cells appear irregular. The pith cells often contain rhomboidal
crystals, and appear to have thicker cell walls than the parenchyma
cells of Xanthorhiza or Coreopsis (Fig. 10c).

[FIGURE 9 OMITTED]

In transverse section, the parenchyma cells of the pith are
generally smaller and more oval shaped closer to the xylem than in the
pith center. The primary xylem consists of a V-shaped cluster of vessels
embedded within a fiber matrix. The pith parenchyma extends past these
wedges to form "arms" on each side of the primary bundles.
Proceeding radially outward on each side of the bundles, the parenchyma
cells make a transition from oval, slightly elongated cells to more
rectangular cells. They have usually assumed the rectangular outline of
typical xylary ray cells by the position where the initial wedge shaped
cluster of vessels merges into the secondary xylem (Fig. 10d). The
parenchyma cells found surrounding the arches of secondary phloem are
the smallest of all parenchyma cells in transverse section.

b. Vessels. Vessels are a prominent feature of the secondary xylem
of Mahonia bealei. They are rounded or oval in cross-section, commonly
with flattened sides where they abut other vessels. The vessels of the
secondary xylem are embedded in a matrix of fibers, in discontinuous
radial strands. These strands contain both single vessels, as well as
groups of vessels in contact with each other. Clusters containing large
numbers of multiple vessels are not uncommon. Vessel groups of 24, 33,
and 63 can be observed. However, single vessels and groups of two or
three vessels also occur. The mean number of vessels per group is 6.7
vessels (n=25: SE=0.3) (Fig. 10e).

[FIGURE 10 OMITTED]

The radial strands of vessels are broken into segments by areas of
the fiber matrix (Fig. 10e). These fiber areas may be fairly large,
producing a large break in the strand, or only a few fibers wide. In
general, the vessels formed just exterior to a break are normally
slightly larger than those farther out in the strand. In many strands,
however, this is not a consistent trend, and changes in vessel diameter
may not be readily apparent. Since there are differences in vessel
diameter, the vessel distribution can be called semi-ring porous, though
no obvious growth rings are evident (Fig. 10d, e).

The discontinuous vessel strands form a zigzag pattern between two
xylary rays, although some sections of a strand may be radially or
tangentially oriented (Fig. 10e). Vessels in a strand fairly commonly
contact one of the rays. Only one or two vessels may be in contact with
a ray, or many vessels may be in contact (Fig. 10e).

The vessel elements have helical secondary wall deposition patterns
and simple perforation plates, with at least moderately oblique end
walls. Often one or both of the end walls have a caudate tip, which are
occasionally extended (Fig. 10f). Simple lateral wall pitting can be
seen on most vessel elements. The pitting is in the form of narrow oval
pits (frequently slit-like) in an alternating pattern that tends to
follow the groove formed between the helices on the lateral wall. The
mean length of vessel elements is 257.5 [micro]m (n=50; SE=9.2), and the
mean width is 24.7 [micro]m (n=50; SE= 1.1). Storying is only
occasionally present (Fig. 7g).

Vasicentric tracheids are present, and are most commonly associated
with smaller vessel elements. They have the same type of lateral wall
deposition pattern and lateral wall pitting as vessel elements, but are
long and narrow in diameter. No measurements were made of their length
or width.

An age-on-length curve fitted with a Locally Weighted Scatterplot
Smoother line shows that vessel element length remains roughly constant
across the xylem. The more recently formed elements have similar lengths
to those of older elements (Fig. 11b). When a linear regression line is
fitted to the same data, a nearly flat line results (p=0.832,
[r.sup.2]=0.00; Fig. 11a).

[FIGURE 11 OMITTED]

c. Rays. Kribs Heterogeneous Type I rays are a prominent feature of
the secondary xylem of M. bealei. They begin as interfascicular regions
of the primary plant body (primary rays), and extend in an unbranched
file of cells through the secondary plant body, widening radially,
before ending outside the phloem (Fig. 10e, h). As seen in transverse
sections, the rays are usually only one or two cells wide at the pith,
but may be three to four cells wide in some cases. Most are
multiseriate, but occasional uniseriate rays also occur. Ray height
could not be determined because they extended beyond the limits of the
tangential sections (5.67 mm) (Fig. 10h). A few small, uniseriate rays
occur where the radial strands of vessels and fibers anastomose (Fig.
10h, arrowhead), but uniseriate rays are not typical and were not
measured. Ray width is relatively constant when seen in tangential
section. Mean ray width is 63.8 [micro]m (n=27; SE=2.9), or 3.7 ray
cells (n=27; SE =0.126) (Fig. 10h). Few of the rays exhibit the convex
shape commonly seen in tangential sections of other species (Fig. 5k).

Although most of the rays are multiseriate (typically 2-4 seriate),
many have uniseriate portions. The alternation between multiseriate and
uniseriate sections occurs in no particular pattern. Often very short
rays are uniseriate for their entire height. The ends of the rays are
also uniseriate, either in the form of a single ray cell, or as an
extended uniseriate tip, five cells or longer (Fig. 10h).

In tangential section, the ray cells may be rounded, oval, or ovoid
in shape. A mix of these shapes occurs in both multiseriate and
uniseriate rays. Often the oval and ovoid shaped cells are found on the
margins of the ray with their longer axes oriented vertically, or in the
uniseriate portions of a ray. None of the rays are storied (Fig. 10h).

In radial section the rays are a mix of both procumbent and upright
cells, with no clear domination by either type. A radial file of cells
usually contains cells of a single shape, but there are exceptions. In
some instances several files of procumbent cells will alternate with
several rows of upright cells, with no discernable pattern of
alternation (Fig. 10i). Procumbent cells have a mean height of 25.5
[micro]m (n=30; SE= 1.0), and a mean width of 46.1 [micro]m (n=30;
SE=2.1). Upright cells have a mean height of 34.0 [micro]m (n=20;
SE=1.4), and a mean width of 31.4 [micro]m (n=20; SE=0.8). The mean
height for all ray cells is 28.9 [micro]m (n=50; SE=1.0), and the mean
width is 40.3 [micro]m (n=50; SE=1.7).

d. Fibers. The secondary xylem of Mahonia bealei contains many
libriform fibers surrounding the vessels and rays, and forming a matrix
in which the other cell types are embedded. The fibers have simple pits
and appear round or oval in transverse section (Fig. 10e). They are
commonly elongated with pointed ends. Mean fiber length is 387.1
[micro]m (n=50; SE=14.5), and mean width is 15.5 [micro]m (n=50;
SE=0.5). Some storying occurs, but is restricted to limited areas of two
to four adjacent fibers.

e. Secondary Woodiness. Plotting woodiness on Kim et al.'s
(2004b) phylogenetic tree of the Berberidaceae yields an equivocal
reconstruction of the ancestral state for Mahonia bealei (Fig. 12).
Mahonia and Berberis (both woody) are sister to the nonwoody Ranzania,
but the presence of a woody outgroup, and their proximity to Nandina
(woody) leads to an ambiguous ancestral state reconstruction. Based on
the character state changes on this tree we cannot determine if their
woodiness was derived from a woody or nonwoody ancestor.

[FIGURE 12 OMITTED]

4. Paedomorphosis in Primitively Woody Taxa

A plot of the co-occurrence of scalariform perforation plates and
Paedomorphic Type III rays on a phylogeny of the Ericaceae (Kron et al.,
2002) suggests that paedomorphic wood has evolved several times
independently in the family (Fig. 13). However, the large amount of
missing data weakens this conclusion, as it is possible that the family
possessed paedomorphic wood as a primitive characteristic, with the
evolution of non-paedomorphic wood occurring several times. In either
case, paedomorphic wood occurs within a primitively woody family and so
cannot be used as prima facie evidence of secondary woodiness.

[FIGURE 13 OMITTED]

This plot, and the data in the Xylem Database (Schweingruber &
Landolt, 2005-2008), also make it clear that the taxa Chimaphila
umbellata and Pyrola rotundifolia, which are normally considered
nonwoody, produce enough secondary xylem to be considered at least
marginally woody. Both species produce secondary xylem with abundant
fibers, and can easily be called woody under most definitions of this
term.

V. Discussion

A. Xanthorhiza simplicissima

1. Vessels

Our results agree with Carlquist (1995a) that the vessels are
distributed in a semi-ring porous pattern, are restricted to the central
part of the fascicular areas, and have no contact with the rays.
However, the mean length of the vessel elements found here (249.4
[micro]m) is longer than the mean length reported by Carlquist (1995b:
167.0 [micro]m), and much shorter than the mean length in dicotyledonous
woods overall (649.0 [micro]m; Metcalfe & Chalk, 1950).
Unfortunately, measures of sample variability are not available in the
literature, so we are unable to determine if these differences are
statistically significant. We also found a difference in mean vessel
element width: 32.9 [micro]m, compared to 20.0 [micro]m reported by
Carlquist (1995a). Finally, the mean number of vessels per group
reported in this study (2.5) is less than that reported in the
literature (4.5; Carlquist, 1995a).

These differences may be a consequence of the samples used or, in
some cases, of the measurement methods. Neither we nor Carlquist used a
true random sampling procedure in selecting stems for study. Carlquist
(1995a) used samples from only one site, while we only sampled three
relatively similar sites. Although working with a limited number of
samples is common in anatomical and systematic studies, it should not be
surprising that occasional differences in measurements are found. While
there is evidence that at least some characteristics of xylem anatomy
(vessel diameter and density, for instance) are affected by
environmental conditions, it is not clear how plastic these
characteristics are, or whether these effects could account for the
observed discrepancies between our and Carlquist's measurements
(Arnold & Mauseth, 1999). If they can account for the discrepancies,
the differences could be due to sampling error. For instance, although
vessel elements in our sample seem substantially longer and wider than
those reported by Carlquist (1995a), they may be within the normal range
of intraspecific variation (Pieter Bass, personal communication). Also,
differences in vessel element lengths might be explained if Carlquist
(1995a) used more mature stems than we did. In paedomorphic woods, mean
vessel element length can decrease over time. It is possible that if
larger, older stems were sampled mean vessel element length might be
shorter. More extensive sampling would be needed to confirm this
hypothesis.

Although we used a different method of measurement than Carlquist
(1995a), this difference cannot account for the observed differences in
vessel diameter. We used wall thickness plus lumen diameter as our
measure of vessel element width, while Carlquist (1995a) used only lumen
diameter. Since he gives wall thickness as 1 [micro]m, even doubling
this to account for the two walls only brings his measurements to 22.0
[micro]m, significantly less that the 32.9 [micro]m (SE=1.6) that we
found. Clearly, differences in method can account for only a small part
of the overall difference in means.

It has been argued that a larger number of vessels per group, and
narrower vessel elements, are advantageous in drier environments
(Mauseth, 1988). Cavitation (introduction of air bubbles into a vessel)
is an increased danger in drier (or colder) areas. A larger number of
vessels per group offers greater protection against this danger through
transport path redundancy. If a vessel in a cluster suffers cavitation,
conductivity can continue in the other vessels. There is, however, no
experimental evidence that greater vessel grouping correlates with
greater cavitation resistance. In contrast, there is a strong
correlation between the area of inter-vessel pitting and vulnerability
to cavitation (Hacke et al., 2006; Wheeler et al., 2005). This
observation complicates the speculative link between vessel grouping and
cavitation resistance. It implies a trade-off between the advantages of
extensive interconnection for redundancy versus the disadvantages for
increasing the probability for air-seeding between vessels. There is
evidence that narrower vessels (which tend to have less pit area per
vessel) may be less prone to cavitation than wider vessels (Sperry et
al., 2006). Thus, vessel groupings composed of narrower vessels would,
hypothetically, be more resistant to cavitation than the same grouping
of larger ones. Although the discrepancy between our measurements and
those of Carlquist (1995a) might be explained if his sample came from a
drier location (implying more vessels per group), the possible validity
of this hypothesis is lessened by the above considerations.

If short vessel element lengths are an adaptation to more xeric
environments as Carlquist has suggested (Carlquist, 1985), adaptation to
a dryer environment might also explain why our measurements of mean
vessel element length are longer then those from his study.

2. Rays

Both we and Carlquist (1995b) found multiseriate rays composed of
mostly upright cells. The rays are Paedomorphic Type II (Carlquist,
2001), and are extensions of the primary rays (Carlquist, 1995b).

3. Fibers

Our descriptions of libriform fibers agree well with those in the
literature. The mean length of xylary fibers reported here is 351.1
[micro]m, compared to a mean of 342.0 [micro]m in Carlquist (1995b).
Though no measures of variability are available for Carlquist's
data, these values would seem to be within the normal range of sampling
error. In both cases, mean fiber lengths are much shorter than the mean
length reported for dicotyledonous woods as a whole (1,317.5 [micro]m;
Metcalfe & Chalk, 1950). When compared with the difference in vessel
element lengths between the studies, the similarity in fiber lengths can
be taken as support for the hypothesis that the differences in vessel
element lengths and widths are adaptive.

The libriform fibers are typically non-storied, and are found
adjacent to the rays and vessels (Carlquist, 1995b). The ratio of
imperforate tracheary element length to vessel element length is 2.05
(Carlquist, 1995b), and 1.4 (this study). These ratios are typical for
species with libriform fibers. If tracheids or fiber-tracheids were
present in the wood, a lower ratio (closer to 1.0) would be expected
(Carlquist, 1995b).

4. Paedomorphosis

Trends in vessel element length as found in the negatively sloped
age-on-length curve, and the presence of upright ray cells, support
Carlquist's (1995b) report of paedomorphic wood in this species.
Although the age-on-length graph shows a negatively sloped curve, there
is only a weak linear association between vessel element length and
distance from the start of the xylem ([r.sup.2]=0.104, p=0.00).

5. Secondary Woodiness

Plotting woodiness on a phylogeny of early diverging dicots (Kim et
al., 2004a), yields three character state changes leading to a nonwoody
habit (Fig. 4). Although one of these took place in a group that
contains Xanthorhiza simplicissima, the presence of woody tissue in
Glaucidium and Hydrastis (Carlquist, 1995b), as well as in the more
basal members of the clade (Nandina, Menispermum, Tinospora) result in
an equivocal ancestral state for Xanthorhiza.

In making this assessment we equated woodiness with the possession
of significant amounts of fibrous secondary xylem. A number of authors
have shown that the possession of secondary growth is much more common
than the possession of fibrous secondary xylem, and is more widespread
than has been traditionally assumed (Dietz & Ullmann, 1997; Dietz
& Schweingruber, 2002; Krumbiegel & Kastner, 1993;
Schweingruber, 2006; Schweingruber, 2007a; Schweingruber & Poschlod,
2005; Schweingruber & Landolt, 2005-2008). Even herbaceous plants
like Dicentra can have more than one secondary growth increment
(Schweingruber & Landolt, 2005-2008). Equating the possession of
wood with the occurrence of secondary growth and replotting what data is
available on the occurrence of secondary growth in the
"herbaceous" taxa (Table 2) does not alter our conclusion,
though it does change which taxa are labeled nonwoody.

6. Environmental Factors and Anatomy

Several of the xylem characteristics of Xanthorhiza simplicissima
are likely adaptations to its environment. In the southeastern United
States, X. simplicissima is most frequently found in the understory of
mature forests, in the moist areas around streams (eFloras, 2008; Rowe
et al., 2004). Although it is found in both the coastal plain and
piedmont, it is most common in the Appalachian Mountains in a belt
running from the deep southern United States (northern Florida and
Texas) into New England (USDA & NRCS, 2009). In this area, the
climate produces clear cut growing seasons where spring and early summer
bring higher temperatures and increased rainfall. Later in the year
lower temperatures halt growth.

Like the ring-porous condition, the semi-ring porous distribution
of vessels is a response to seasonal fluctuations in rainfall and
temperature. The relatively low number of vessels per group indicates
that any added safety of redundant vessels is not needed in the moist,
shaded conditions where X. simplicissima grows. Given conditions of
heavy shade and moist soil, it is likely that high vessel conductivity
is not required to maintain transpiration rates. Another indication that
transpiration may not be excessive is that the leaf surface area of X.
simplieissima is fairly small. This condition may be correlated with the
fact that mean vessel width is much lower than that of dicotyledons as a
whole (Metcalfe & Chalk, 1950).

The paedomorphic features of X. simplicissima--a negatively sloped
age-on-length curve, and the presence of rays with upright cells--may
indicate an adaptation to providing an intermediate amount of mechanical
strength. Although the presence of libriform fibers and the pitted
lateral wall deposition pattern of the vessels create a dense wood, the
rays and short vessel elements tend to offset this increased strength
(Carlquist, 2001). Perhaps the greatest factor affecting mechanical
strength is the limited amount of secondary growth. With limited
secondary growth, stem diameter is not significantly increased (Rowe et
al., 2004). The fact that many of the plants seen in the field are
somewhat procumbent supports this view. The hypothesis that
paedomorphosis represents a relaxation of selection for mechanical
strength is discussed more fully below (Carlquist, 2001).

B. Coreopsis gigantea

There is less anatomical data available on Coreopsis gigantea than
Xanthorhiza simplicissima. What is available is largely in agreement
with our results.

1. Vessels

Vessels in C. gigantea are distributed in a semi-ring porous
pattern, and tend to be storied. Carlquist (1985), reports semi-ring
porous wood, (2) but does not discuss storying. The mean length and
width of the vessel elements in this study is 180.2 [micro]m, and 51.6
[micro]m respectively. The mean number of vessels per group is 1.7.
Measurements from the literature are 210.0 [micro]m and 36.7 [micro]m
for length and width, respectively, and of 2.2 vessels per group
(Carlquist, 1985). Whether these differences are statistically
significant cannot be determined due to the lack of variability measures
reported in the literature. As in Xanthorhiza simplicissima, neither we
nor Carlquist (1985) used true random samples. As a result, sampling
error could play a role in accounting for the different measurements.

A second possibility to account for the differences involves the
age-on-length curves. Two of the three vessel element length data sets
show a drop in vessel element length as the distance from the pith
increases. If our samples came from larger, older stems as compared to
those of Carlquist (1985), then we would expect our measurements to
yield a shorter mean length. However, no details on the age or size of
the plants sampled by Carlquist (1985) are available.

As discussed above, differences in methodology in measuring the
vessel element widths can account for only some of the difference in
mean vessel element width between the two studies. Carlquist (1985)
measured vessel diameter on the basis of the lumen, while we measured
the lumen plus wall thickness. This discrepancy explains part, but not
all, of the difference in mean vessel element width found in the two
studies. Vessel wall thickness would have to be substantial to account
for the total difference in means.

A final possibility to account for the difference in means is the
source of the samples. Carlquist's (1985) plants were field
collected, while ours came from the Berkeley Botanical Garden.
Cultivated plants usually have more mesomorphic characteristics than
wild material of the same species (Bissing, 1982). However, known
differences of this type are not sufficient to account for the
difference in means observed here (Bissing, 1982). The inconsistent
trends among the measurements of vessel size and distribution also do
not support the hypothesis that the samples we used were from a more
mesic habitat. Our mean vessel element width is greater and mean vessels
per group is lower than the same parameters measured by Carlquist
(1985), which is consistent with a more mesic habitat for our samples.
However, our mean vessel element length should be longer than
Carlquist's (1985), and it is not.

All vessel elements seen in this study have simple perforation
plates. There is, however, some variability in lateral wall deposition
patterns, related to the age of the vessels and whether they are located
in the primary or secondary xylem. Vessels with helical wall deposition
patterns are found in the primary xylem, while helical transitional to
scalariform secondary wall deposition patterns are found in later formed
vessels of the secondary xylem. There is no lateral wall pitting in the
conventional sense of scalariform, transitional, opposite, or alternate
pits. The term pseudoscalariform pitting may better describe the type of
pitting found in this species. In pseudoscalariform pitting, smaller
pits are interspersed with other wider pits, many of which extend around
the circumference to more than one wall facet (Carlquist, 2001).

The literature is mixed in its support of the vessel element
characteristics we found in C. gigantea. Carlquist (1974) found
scalariform, or near scalariform, lateral wall pitting in the vessel
elements. He later reported the predominate occurrence of simple
perforation plates, but found no helical sculpturing (defined as
secondary wall thickening connecting the pit apertures) as a secondary
wall deposition pattern (Carlquist, 1985).

This difference can probably be ascribed to the confusing
terminology used to describe vessel element secondary wall deposition
patterns, and vessel element secondary wall pitting. If we exclude
transitional forms (which are common), secondary wall deposition
patterns can be categorized as annular, helical, scalariform,
reticulate, and pitted (Evert, 2006; Mauseth, 1988). In these forms, the
area of the primary wall not covered by the patterned secondary wall is
available for diffusion. Lateral wall pitting can be in a scalariform,
opposite, or alternate pattern, and the pits can be bordered or
nonbordered. With the exception of the pitted secondary wall deposition
pattern, pitting is usually described independently of the wall
deposition patterns (Evert, 2006; Mauseth, 1988).

While the terms lateral wall pitting and lateral wall deposition
pattern describe two different things, they may occur together. For
example, it is conceivable that a vessel element can have a helical
secondary wall deposition pattern, as well as alternate pits. In this
case the primary wall is exposed at the pit apertures, which lie in an
alternating pattern. Additional secondary wall material overlays this in
the form of a raised helical band. This pattern of lateral wall
deposition and pitting can be seen in Mahonia bealei (Fig. 10f).

The common usage of the term scalariform in the literature tends to
conflate the terms scalariform secondary wall deposition pattern and
scalariform pitting. Strictly speaking, these are two different
concepts. A scalariform secondary wall deposition pattern refers to the
raised deposits of secondary wall material. In contrast, scalariform
secondary wall pitting refers to the pattern of the pits themselves.
Often, however, these terms are used interchangeably. In the case of C.
gigantea, the literature probably describes the same phenomenon in
vessel element characteristics as our research, but in different terms
(Carlquist, 1985).

2. Rays

In smaller diameter stems, only a limited amount of xylem is
produced from the fascicular cambium. As a result, the primary rays
(interfascicular regions) are wide. The parenchyma cells of these rays
divide to keep up with the production of xylem by the fascicular
cambium. Over time, interfascicular cambium forms in the parenchyma of
the primary rays, and eventually merges with the fascicular cambium to
form the vascular cambium (Mauseth, 1988). The vascular cambium then
produces the secondary xylem and phloem in a continuous cylinder, which
is clearly visible in larger stem sections.

In C. gigantea the xylary rays appear more as interfascicular
regions in the secondary xylem than as typical rays. In more typical
woody dicots, rays appear in transverse section as radial bands of
parenchyma cells with clearly delineated borders, set in a matrix of
other cells such as libriform fibers. In C. gigantea libriform fibers
are not extremely common, and ray parenchyma often occurs immediately
adjacent to the vessel elements of the secondary xylem. As a result, the
borders of the rays are less defined than those in fibrous woods. Rays
with very: diffuse borders have also been reported in the annual
Calepina irregularis (Brassicaceae) (Schweingruber, 2006).

In transverse section the secondary rays are fairly wide. This is
due in part to the narrow strands of vessels, and the limited amount of
associated fibers produced in the secondary xylem. The width of the
secondary rays is also due to the fact that they are the continuation of
wide primary rays (Mauseth, 1988).

3. Fibers

There is no comparable information on libriform fibers available
from the literature.

4. Paedomorphosis in the Secondary Xylem

The data and observations reported here support the hypothesis that
the secondary xylem of C. gigantea is paedomorphic. The age-on-length
curves, pseudoscalariform pattern of secondary lateral wall pitting, and
uptight ray parenchyma cells, all provide evidence of paedomorphosis
(Carlquist, 1962, 2009).

Both negatively sloped and flat age-on-length curves were found in
C. gigantea. The fact that we found both negatively sloped and flat
lines may be a result of the fact that little of the variation is
explained by the regression lines. The [r.sup.2] values (0.101, 0.175,
0.001) make it obvious that there is no strong linear association
between vessel element length and the distance from the start of the
xylem. It may be that we were unable to detect a negative slope in the
second sample from the 58.0 mm stem because of this, or because our
second set of measurement only covered 6.2 mm of xylem instead of the
8.0 mm covered by the first sample. Alternatively, it is possible that
reports of the existence of two types of paedomorphic curves are in
error (Carlquist, 1962; Carlquist, 1989; Carlquist, 2001; Carlquist,
2009; Lens et al., 2009). The high amount of variability in vessel
element lengths may make it impossible to differentiate negatively
sloped from flat curves. Sampling stems of different size classes in a
single species, along with publication of measures of goodness of fit,
are needed to provide evidence to distinguish between these hypotheses.
In any case, the vessel element length data in all three graphs indicate
paedomorphosis.

5. Secondary Woodiness

There is little discussion in the literature on whether Coreopsis
gigantea is woody, or possesses secondary woodiness. Carlquist considers
it as an example of a woody herb, and lists it in his catalogue of woody
herbs on islands (Carlquist, 1974). He also includes it as a member of
the woody genera in the Munz flora of southern California (Carlquist,
1985; Munz, 1974). Thorne also lists it as an example of insular
woodiness (Thorne, 1969).

Our character mapping on Mort et al.'s (2004) phylogeny of the
western clade of Coreopsis shows that C. gigantea is secondarily woody,
but the ancestral state for the genus remains in doubt (Fig. 9). Higher
level phylogenies of the whole genus, and of tribe Heliantheae, help
resolve this question. Kim et al. (1999) constructed a phylogeny of
Coreopsis based on ITS sequences. In it, they used two outgroups basal
to Coreopsis: Fitchia speciosa, and Dahlia coccinea/Dahlia macdougallii.
Fitchia speciosa is a tree (USDA & NRCS, 2009), and is basal to
Dahlia. Dahlia is nonwoody (Harris, 2008; Saylor, 2008; Vivar-Evans et
al., 2006). This study suggests that the ancestor of Coreopsis lacked
wood. A more recent study used nuclear 18S-26S rDNA internal transcribed
spacer (ITS) sequences to construct a phylogeny of the Heliantheae, the
tribe that contains Coreopsis (Baldwin et al., 2002). This study also
supports an ancestral perennial, herbaceous state for Coreopsis (Baldwin
et al., 2002).

The use of the term woody to describe the mature stems of Coreopsis
is somewhat problematic, especially in light of our use of this term to
refer to secondary xylem with a high percentage of fibers. The secondary
xylem of C. gigantea shares more similarities with the wood of so-called
herbaceous perennials then to the secondary xylem of highly fibrous
woody species (Dietz & Ullmann, 1997; Dietz & Schweingruber,
2002; Krumbiegel & Kastner, 1993; Schweingruber, 2006;
Schweingruber, 2007a; Schweingruber & Poschlod, 2005; Schweingruber
& Landolt, 2005-2008). We return to this problem in the general
discussion.

6. Environmental Factors and Anatomy

Coreopsis gigantea is found on the Channel Islands of southern
California, and along a narrow strip of the mainland bordering seven
counties in central and southern California (Monterey, San Luis Obispo,
Santa Barbara, Ventura, Los Angeles, Riverside, San Diego; Consortium of
California Herbaria, 2008). It usually grows on cliffs and sand dunes,
exposed to the effects of the nearby ocean. In general, this area has a
Mediterranean climate. Temperatures are mild and relatively stable, but
there are seasonal differences in rainfall. Winters are wet, while
summers tend to be dry (Keil, 1993; Ritter, 2006; USDA & NRCS,
2009).

Some of the xylem characteristics of Coreopsis gigantea seem to be
an adaptation to this climate and habitat. While its vessels have a
semi-ring porous distribution, it can be difficult to distinguish larger
diameter vessels from smaller vessels in transverse sections. True
growth rings bordered by a complete lack of vessels or the presence of a
terminal fiber band are lacking. Instead, there are breaks in the radial
vascular strands indicating seasonal disruptions of growth.

The low number of vessels per group may also be an adaptation to
mesic conditions. It is possible that the large amount of parenchymatous
tissue helps to store water, lessening any adaptive value of large
vessel groups, which may be advantageous in xeric environments
(Carlquist, 1985; Mauseth, 1988). Its shallow root system, fern-like
leaves, and moderate vessel widths also indicate a lesser need for the
high vessel conductivity that accompanies high transpiration rates. All
of these factors are consistent with an adaptation to a mild climate
without strong seasonality.

Of the three species studied here, C. gigantea has the highest
number of paedomorphic features. Several of its xylem characteristics
support the hypothesis that paedomorphosis represents a release from
selection for mechanical strength (Carlquist, 2001). Although branches
do occur, C. gigantea is not a tall plant, and its shoots do not appear
to offer much wind resistance. As a result, its need for mechanical
strength would seem to be minimal. The abundant parenchyma in its stem
offers sufficient turgor to support the stem, without the need for
extensive support from sclerenchyma. In conventional woods, rays offer
less strength than the fibers they displace, and may weaken the wood.
However, in C. gigantea, fibers are uncommon and rays may provide all
the strength that is needed.

C. Mahonia bealei

Our results for Mahonia bealei are in general agreement with
published findings on this species (Carlquist, 1995a).

1. Vessels

The mean length (257.5 [micro]m) and width (24.7 [micro]m) of the
vessel elements reported here are comparable to the published means for
this species (272.0 and 13.0 [micro]m, respectively; Carlquist (1995b).
We found a mean of 6.7 vessels per group, compared with a mean of 11.8
vessels per group reported in the literature (Carlquist, 1995a).

The difference in vessel lengths can almost certainly be attributed
to sampling error, though measures of variability are not available from
the literature to verify this. The lower mean width of the vessel
elements can be partially explained by the fact that Carlquist (1995b)
only measured the lumen. If we estimate the combined thickness of the
two cell walls as 2.0 [micro]m, and subtract this amount from our
measurements, it brings our measurements to 22.7 [micro]m, at least a
bit closer to Carlquist's (1995b) figure of 13.0 [micro]m. The
remaining difference in vessel width could easily be due to
intraspecific variation or, if the differences are statically
significant, could, along with the higher number of vessels per group
indicate greater xeric adaptation; although we might also expect to see
shorter vessel element lengths in xeric adapted plants. This hypothesis
may be supported if Carlquist's collection was from the field (ours
was cultivated), though the greater availability of water to cultivated
plants may not necessarily lead to greatly increased mesomorphic
characteristics (Bissing, 1982).

2. Rays

There are no reports of ray cell dimensions in the literature.
Carlquist (1995b) estimates ray height as 2.4 mm based on tangential
sections. This seems low as our maximum section height, which the rays
exceed, was 5.67 ram. Mean ray width is 63.8 [micro]m, or 3.7 cells.
This compares to a ray width of 4.4 cells reported in the literature
(Carlquist, 1995a).

3. Fibers

Mean fiber length is 387.1 [micro]m, and mean fiber width is 15.5
[micro]m. This is shorter than the mean length of 541.0 [micro]m
measured by Carlquist (1995b). Although we found limited fiber storying,
our results can easily be reconciled with Carlquist's (1995b)
report of no fiber storying. The difference is likely due to a
difference in interpretation. Fiber storying is certainly rare.

4. Paedomorphosis

Mahonia bealei has the fewest paedomorphic features of the three
species studied here. As a result, it has only a minimal degree of
paedomorphosis. The only indication of paedomorphosis is the flat
age-on-length curve of vessel element lengths. Since [r.sup.2]=0.00
(p=0.832) none of the observed variation is explained by this line,
which is not significantly different from zero.

The weak paedomorphosis found in Mahonia bealei can perhaps best be
understood in light of the hypothesis that paedomorphosis represents a
relaxation of selection for mechanical strength. Mahonia bealei has
limited branching, can grow up to 4 (-8) meters tall (Ying et al.,
2010), and can be top heavy (Fig. 2c). This habit requires more
mechanical strength than do the habits of the other species investigated
here. Attempts to section its stems demonstrate the density of its wood.
All of these facts support the hypothesis that there is an inverse
relationship between degree of paedomorphosis and mechanical strength
(Carlquist, 2001).

5. Secondary Woodiness

Loconte and Estes (1989) produced a morphological phylogeny of
Berberidaceae that shows a woody ancestry for Berberidaceae, then a
shift to herbaceousness, with a reversion to woodiness in the
Berberis/Mahonia branch. They support their conclusion that Berberis and
Mahonia are secondarily woody by citing the apomorphic characteristics
of the wood anatomy: ring porosity, little or no wood parenchyma, and
homogenous, uniseriate rays (Loconte & Estes, 1989; Shen, 1954).

The rbcL maximum-parsimony tree generated in Qiu et al.'s
(1993) study of the Magnoliidae places Papaverales basal to
Ranunculales. The Papaverales are usually reported to consist of
herbaceous plants (Dicentra and Papaver were used as representatives of
the order in the study). Since the Ranunculales contains many woody
genera (Mahonia, Xanthorhiza, Cissampelos, Coeeulus, Akebia, and
Euptelea are included in the tree, along with Caltha, which is
herbaceous), the implication is that Mahonia has herbaceous ancestry
(Qiu et al., 1993). However, recent work has shown that both Dicentra
and Papaver have secondary growth (Schweingruber & Landolt,
2005-2008). Dicentra formosa produces distinct rings of secondary xylem,
as do Papaver alpinum, P. auranthiacum, and P. variegatum. Papaver
dubium, P. rhoeas, and P. somniferum are annuals that produce a single
ring of secondary xylem (Schweingruber & Landolt, 2005-2008). These
findings weaken the case for the herbaceous ancestry of Mahonia.

Judd et al. (2002) lists several apomorphies for the genus
Berberis, including secondary woodiness (Judd et al., 2002). Though they
do not discuss Mahonia, the proximity of Mahonia to Berberis on Loconte
and Estes' (1989) tree suggests that Judd et al. (2002) would also
consider it secondarily woody.

Kim and Jansen (1998) used chloroplast DNA restriction sites to
construct the phylogeny of 16 genera of the Berberidaceae. They
recognized four basic groups in the family, corresponding to the four
groups based on base chromosome number of 6, 7, 8 and 10. These same
groups were also recovered in an earlier study (Kim & Jansen, 1996).
Kim et al. (2004b) used the chloroplast gene ndhF to further investigate
the phylogeny of the family in the hope of resolving the relationships
between the chromosomal groups discovered earlier (Kim & Jansen,
1998). Though they failed in this goal, they were able to provide strong
support for retaining Nandina in the Berberidaceae. The retention of
Nandina is relevant because it has woody stems, and because its position
on the tree influences our character state reconstruction.

Our character state mapping on Kim et al.'s (2004b) phylogeny
was unable to resolve the phylogenetic history of woodiness in Mahonia
and Berberis (Fig. 12). Plotting the same data on Kim and Jansen's
(1998) phylogeny suggests that these taxa are primarily woody. The
difference in these conclusions is due largely to the position of
Nandina in the two studies. Kim and Jansen (1998) place it as sister to
the rest of the family, whereas in Kim et al.'s (2004b) study it is
strongly supported as the sister group to the clade that contains
Caulophyllum, Leontice and Gymnospermium (Fig. 12). Phylogenetic
reconstructions of character state history are strongly dependent on the
shape of the underlying phylogeny, and on the character states assigned
to the terminal taxa (Olson, 2007).

6. Environmental Factors and Anatomy

The genus Mahonia is native to the Pacific Northwest of the United
States, as well as Central America and Asia. Although M. bealei is used
widely as a landscaping shrub throughout the United States, it
originated in China. Its semi-ring porous pattern of vessel distribution
suggests that it is adapted to a climate with a clearly delineated
growing season. We assume, of course, that these characters are not so
plastic that they have been influenced by the local, North Carolina
climate. Although trends in the vessel strands are not always
consistent, the first part of the growth ring consists of noticeably
larger diameter vessels, which means growth is initiated with higher
temperature and plentiful water. Later in the ring the vessel diameters
are smaller, which may indicate an adaptation to less rainfall.

Mahonia bealei has a higher number of vessels per group than in
either Xanthorhiza simplicissima or Coreopsis gigantea. This, together
with the presence of vasicentric tracheids and fairly narrow vessels,
may provide a margin of safety in case of fluctuations in water supply.
Narrow vessels and tracheids have been shown to resist cavitation better
than wide ones (Hacke & Sperry, 2001; Sperry et al., 2006). Its
vessels also have helical lateral wall deposition patterns, which may
allow for some deformation of the vessel walls. This feature could help
prevent implosion of the vessels due to extreme negative pressure
resulting from dry conditions.

D. Paedomorphosis in the Secondary Xylem

Carlquist (2009) recently placed paedomorphosis in the secondary
xylem in the context of heterochrony in the xylem of angiosperms.
According to his synthesis, there is a continuum of heterochronic events
in the xylem. On one end of the continuum, plants like the monocots
Gunneraceae, Nelumbonaceae, and Nymphaeaceae have lost cambial activity,
resulting in a permanently juvenile xylem. Juvenile features of these
plants include sympodial, less woody growth forms, decreasing vessel
element lengths across the primary xylem, and raylessness (or
exclusively upright ray cells). On the opposite end of the continuum are
typical woody plants like trees and shrubs. These plants have an active
vascular cambium that produces secondary xylem with mature (adult)
characteristics very early in life. They have woody monopodial growth
forms. Their vessel elements decrease in length in the primary xylem,
but then increase in length in early secondary xylem before leveling off
in later secondary xylem. Ray cells tend to be mostly or exclusively
procumbent. Plants with paedomorphic secondary xylem fall between these
extremes, but lie towards the monocot end of the spectrum. They possess
at least some of the paedomorphic characters discussed in the
Introduction (Carlquist, 2009).

1. The Baileyan Trends

Carlquist based part of his theory of paedomorphosis on
Bailey's (1944) refugium theory (Carlquist, 2009). Bailey (1944)
proposed that advanced wood features, by which he means features adapted
for maximum conductance, first evolved in the secondary xylem, and then
later appeared in the primary xylem. This evolutionary scenario explains
why the primary xylem tends to retain more primitive (less optimal)
tracheary elements (Bailey, 1944; Carlquist, 1962; Mauseth, 1988).
According to Carlquist's (1962) paedomorphic theory, anatomical
features that occur in the primary xylem of typical woody plants also
occur in the secondary xylem. Therefore, in paedomorphic wood, some of
the features of the secondary xylem will be primitive, in the Baileyan
sense (Carlquist, 1962).

Since the Baileyan trends serve as an underpinning to
Carlquist's (1962) theory, it is good to at least briefly examine
possible weaknesses in the assumptions behind these trends. Although
Carlquist (1962) uses the trends as a point of reference, his theory
actually serves as a partial refutation of them.

All the major Baileyan trends were discovered either by the use of
the fossil record to determine the timing of a character's
appearance, or through correlations with the primitive features so
discovered (Bailey, 1944; Bailey & Tupper, 1918; Frost, 1930a, b,
1931; Kribs, 1935, 1937; Tippo, 1938). Bailey's general trends were
confirmed by Wheeler and Baas (1991) through a study of the frequency of
woody character state occurrences in a database of 1200 fossil woods.
Unlike a previous survey (Chalk, 1937), Wheeler and Baas (1991) were
able to relate their observed frequencies to geological periods.
Although not all of Bailey's trends were supported (septate fibers
are more common in the fossil record than Bailey would have predicted),
most were.

A problem with using correlation methods in these ways is that they
may not be tracking changes in the characters themselves, but changes in
the relative abundance of taxonomic groups in the fossil record (or
sampling errors in the database). For instance, the low percentage of
species with short (< 350 [micro]m) vessel elements in the Cretaceous
may be an indication of the composition of the flora, not of the
primitive nature of short elements (Wheeler & Baas, 1991). Warren
(Herb) Wagner long ago recognized this problem, and warned against it
(Wagner, 1969). Neither Chalk (1937) nor Wheeler and Baas (1991)
summarized the taxonomic distribution of the fossils, which would at
least partially address this criticism. In some cases the taxonomic
identity of the fossils was not even known (Wheeler & Baas, 1991).
These types of methods would have greater validity if they were
conducted within a phylogenetic context, which would address the problem
of species abundance.

Although the Baileyan trends are still accepted as reasonable
descriptions of the general course of xylem evolution (Evert, 2006),
some of their underlying assumptions limit their usefulness. The fact
that the trends were developed outside of a taxonomic framework, and
were therefore treated as irreversible, was originally seen as one of
their strengths (Tippo, 1938). Homoplasies in the trends are now known,
with the instance of parallelisms outnumbering reversals by a factor of
two (Baas & Wheeler, 1996). The existence of homoplasies makes it
necessary to test the assumed primitive conditions implied by the trends
in the context of each new study. They can no longer be considered
irreversible.

The assumption that water conduction was the major driving force
behind the evolution of vessels has also been questioned (Mauseth, 1988;
Sperry, 2003). It is now more common to view the different types of
tracheary elements as having evolved in response to the environmental
and physical demands put on the particular vascular tissue (Mauseth,
1988). Plants with only primary xylem grow under different conditions,
and have different physiological needs than older plants of the same
species that have developed secondary xylem. Since different demands are
put upon their vascular systems, the tracheary elements must assume
different structural forms, and must have evolved to meet these demands.
For example, younger plants may be shorter, so that root pressure may be
sufficient to move water and minerals up through the xylem. In this
case, tracheids and narrower vessels may suffice. Older, taller plants
are dependent on cohesion-tension to move water, and may need wider
vessels to meet their water transportation needs, and longer vessel
elements to provide additional structural strength (Mauseth, 1988).

There are, of course, tall plants like the conifers that do not
rely on wide vessels to meet their water requirements. Instead, they use
tracheids as water conduits. However, since conifers are often found in
habitats with limited resources (such as areas of low soil fertility),
the lower resource requirements of their xylem may offer an advantage
that outweighs conductivity. Thus tracheids, which are found in less
dense woods that require less energy and less structural compounds to
construct, are able to meet both the conductive and resource needs of
conifers (Hacke & Sperry, 2001; Sperry, 2003).

Ecological wood anatomy has also shown that certain environmental
factors like seasonality, nutrient availability, and temperature have
strong correlations with wood structure (Endress et al., 2000). In
experiments with the cacti Cereus peruvianus and C. tetragonus,
reduction of vessel density and vessel diameter was triggered by
reducing the availability of nitrogen and phosphorous in the soil
(Arnold & Mauseth, 1999). While these may be only short-term
responses, it seems reasonable that long-term selection pressure could
reverse at least some of the major evolutionary trends.

Functional studies have provided additional insights into xylem
evolution. For example, there are inherent trade-offs between
conductivity and safety. Wider vessels may offer greater conductivity,
but increase the risk of cavitation through the formation of bubbles in
the water column from freeze-thaw cycles, or from drought (Hacke &
Sperry, 2001). Larger lateral wall pits also enhance conductivity, but
weaken the vessel wall. The extent of lateral wall pitting may even be
more important than lumen diameter in safety issues (Sperry, 2003;
Sperry et al., 2006). The increased porosity of the pit membranes also
offers increased conductivity, but this increases the risk of air
seeding, which causes cavitation (Sperry, 2003). Thicker vessel walls
offer greater safety against collapse caused by negative pressure;
however, there is a greater cost to the plant since there is a greater
investment in wall material (Sperry, 2003).

Transpiration rate is another driving force behind the evolution of
tracheary elements. Most of the water taken in by plants is lost due to
transpiration that occurs during the gas exchange associated with
photosynthesis. Transpiration rate can be affected by several factors
including size of the leaves, the number of stomata, water availability,
and C[O.sub.2] concentration in the atmosphere (Sperry, 2003). There is
evidence that early plants did not need highly conductive tracheary
elements because of the much higher C[O.sub.2] levels at that time. The
highly favorable exchange rate for C[O.sub.2] versus water greatly
reduced the cost of transpiration. These factors might at least
partially explain the tracheary element morphology seen in early fossil
plants (Feild et al., 2004). Feild et al. (2004) also argue that these
ancestral characters were adapted to the high water availability and the
low evaporative demand in the tropical, rainy environments of the early
plants.

Although these considerations weaken the validity of the Baileyan
trends, the trends still offer some insight into xylem evolution.
Bailey's refugium theory, however, is much weaker and does not
adequately explain how primary xylem and secondary xylem differ. The
primary xylem can only be described as an evolutionary time capsule if
the trends are irreversible, or if each of Bailey's primitive
states could be shown to be primitive for the angiosperms as a whole. In
the latter case, the trends would be similar to morphological transition
series: multistate characters with restricted transitions between the
states (Swofford and Begle, 1993). Characters like this are hypotheses
of evolutionary transformations that must be tested through correlations
with other characters in a parsimony framework. These types of analyses
would likely show that in some taxa the primary xylem retains many
primitive states, while in others it does not. The structure of the
primary xylem is shaped both by the selective pressures placed on the
young plant (Mauseth, 1988), and by historical, lineage specific events.
During the period when the primary xylem serves as the main conductive
tissue, young plants frequently have lower transpiration demands. They
may be shaded in the understory, or have smaller masses of leaves with
correspondingly fewer stomata. They are also smaller, so that the fibers
and heavier vessel element lateral wall deposition patterns seen in the
secondary xylem are not needed for structural support (Mauseth, 1988).
These requirements may differ in different taxa, leading to different
structures of the primary xylem.

Research on the genetic and cellular mechanisms behind secondary
growth also provides insight into the linkage between the primary xylem
and the secondary xylem. Plants like Arabidopsis thaliana that normally
have only primary growth can be induced to produce secondary xylem
(Nieminen et al., 2004). It has also been shown that the genes
responsible for the production of secondary xylem in loblolly pine have
homologs in Arabidopsis. Thus, the primary xylem has structural
differences from secondary xylem partly because of the different
expression of the same gene families (Groover, 2005; Nieminen et al.,
2004).

2. Mabberley's Criticism of Paedomorphosis

Mabberley (1974) disagreed with Carlquist's (1962) use of
age-on-length curves for vessel elements as a basis for his theory of
paedomorphosis. Mabberley noted that plants like Talinum guadalupense,
with a negatively sloped age-on-length curve, and Macropiper excelsum,
with a flat age-on-length curve, are pachycauls. Pachycaulous plants
have thick, often parenchymatous, stems with piths that become
progressively wider higher in the stem. Mabberley argued that even if
the vessel element measurements that produced the curves were taken at
the same height, they were not taken from geometrically equivalent parts
of the plants. This necessarily produces different types of curves due
to the differences in pith radii in the pachycaulous and leptocaulous (a
typical tree or shrub) plants (Mabberley, 1974).

Basic geometry illustrates why different curves will be produced
from plants with different pith radii. The increase in the amount of
xylem is dependent on two factors: the number of cambial initials
available for division, and the rate of cell division. The number of
cambial initials available for division is given by the formula
(27[tau]r)/x, where x=the tangential width of cambial initials, r=the
radius of the pith and secondary xylem, and 2[tau]r=the circumference of
the vascular cambium. As is apparent from this formula, the smaller the
radius, the smaller the circumference, and the fewer the cambial initial
cells available for division (Mabberley, 1974). Under these conditions
the rate of anticlinal cell division becomes important in increasing the
circumference of the cambium. If the rate of anticlinal division cannot
keep up with the periclinal divisions that add cells to the secondary
xylem, then the increased circumference must be made up by apical
intrusive growth of the initials (Evert, 2006). Intrusive growth
increases the number of initials available in the circumference by
causing initials from a lower level to intrude between the initials of a
higher level. Later transverse divisions in the elongated initials
results in two shorter cells, where there was originally only a single
long cell (Mabberley, 1974). As the radius of the stem increases and
these processes continue, there are more cambial initials available for
radial anticlinal division, and the need for intrusive growth is
reduced. As a result, when vessel element length is plotted against
distance from the center of the stem there is a steep rise in vessel
element length in the earlier formed secondary xylem, then a reduction,
and eventually a plateau (Mabberley, 1974).

Pachycaulous plants like Macropiper excelsum and Talinum
guadalupense have wide piths, that become wider upwards. The greater
radius means that there are more cambial initials available for
anticlinal division, so it is easier for these divisions to keep up with
the increase in secondary xylem (Mabberley, 1974). Therefore, there is
less need for intrusive growth to increase the number of initials, and
vessel element lengths do not increase. Since these plants start with a
wide pith, a plot of their vessel element lengths resembles the latter
part of the age-on-length curves for a typical woody plant. However, if
vessel elements were measured at the base of a pachycaulous plant, the
age-on-length curve would resemble the full curve (Mabberley, 1974).

Since the age-on-length curves produced by the paehycaulous plants
in Carlquist's (1962) study can be explained by the wider pith at
the location where the measurements were taken rather than by
paedomorphosis, Mabberley (1974) argues that the theory of
paedomorphosis is not supported by the curves. Measurements of vessel
element lengths at geometrically equivalent locations are necessary to
support the theory (Mabberley, 1974).

The use of wood characteristics, either with or without
paedomorphosis, to determine the ancestry of taxa (Lens et al., 2009;
Lens et al., 2005a) can be problematical. Evolutionary relationships
among genera can be difficult to uncover, and simple morphological
assessments used outside of a phylogenetic context have limitations in
reconstructing the ancestral origins of woodiness. This is especially
true in insular woody genera, where problems in determining whether a
species was introduced by single or multiple colonization events, the
time of radiation, and the continental relatives and geographic sources
of the original colonizers are especially difficult (Kim et al., 1996).

The evolution of woodiness in Tolpis (Asteraceae) provides a
cautionary tale on determining the direction of evolution of woodiness
in general, and insular woodiness in particular. Tolpis is a genus of
ca. 16 species in Macaronesia and the nearby continents of Europe and
Africa. All but two of these have woody stems. The two nonwoody species
are annual herbaceous plants: Tolpis coronopifolia (Macaronesia, Canary
Islands) and Tolpis barbata (Mediterranean area of Europe and Africa).
Both of these species can grow as biennials under certain environmental
conditions, in which case they occasionally develop woody stem bases
(Moore et al., 2002).

Moore et al. (2002) constructed a phylogeny of Tolpis based on
chloroplast DNA restriction site variation. Restriction site mutants
were coded as absent or present, and parsimony analyses were done to
generate phylogenetic trees. The geographic distribution of the extant
species was then mapped on each clade, resulting in nine equally
parsimonious reconstructions of dispersal history (Moore et al., 2002).

The topology of the strict consensus tree shows that woodiness is
plesiomorphic in Tolpis. Tolpis has primary woodiness, while the annual
herbaceous habit evolved twice, independently. The two herbaceous
species are derived from woody ancestors (Moore et al., 2002).

The nine equally parsimonious reconstructions of dispersal history
fall into two groups. The first group, consisting of eight
reconstructions, implies that Tolpis colonized the Madeira Islands from
the continent, followed by extinction of the genus on the continent, and
then subsequent continental recolonization from Macaronesia. There is
little agreement among the reconstructions on which island group in
Macaronesia is the source of the existing continental species. The
second group, consisting of the ninth reconstruction, requires four
separate introductions of Tolpis from the continent.

To refine the analysis, minimum geographic distances required for
travel between the regions were assigned to each dispersal event in each
of the nine scenarios. Ranking the nine reconstructions by minimum
distance required favors the first group of reconstructions, those
involving continental extinction and recolonization (Moore et al.,
2002).

While the authors make it clear that the data does not exclude the
possibility of the evolution of woodiness in Macaronesia (it only
supports the plesiomorphy of woodiness in the extant taxa), a natural
interpretation of their results is that the initial colonists to
Macaronesia were woody. If any of the eight, group one, dispersal
reconstructions are correct, then the extant herbaceous continental
species T. barbata evolved from woody Macaronesian ancestors (Moore et
al., 2002).

It is intriguing that a woody continental species of Tolpis may
have colonized Macaronesia, with resulting speciation throughout the
archipelagos, extinction of the woody continental species, followed by a
recolonization of the continent by an herbaceous species derived from a
woody Macaronesian ancestor. Contrast this scenario with what would be
more natural to assume based only on the distribution of woody and
nonwoody taxa. Given the current distribution, it would be natural to
assume that an herbaceous species from the continent colonized
Macaronesia, and that woodiness evolved as the species spread throughout
the islands. After all, the extant continental species of Tolpis is
herbaceous (T. barbata), while all but one of the Tolpis ssp. in
Macaronesia are woody. The results of this study make it evident that
the evolution of woodiness can be complex, and that its study requires
more than knowledge of the distribution of woody and nonwoody taxa.

A second example illustrates the complexities involved in the
evolution of woodiness, and the evolution of insular woodiness in
particular. Eleven species of Convolvulus (Convolvulaceae) are endemic
to Macaronesia (the status of a twelfth species, the weedy annual C.
siculus, is uncertain). All are woody. The eleven endemic species fall
into two distinct groups, climbers and non-climbers (Carine et al.,
2004).

Carine et al. (2004) used data from the nuclear ribosomal internal
transcribed spacer (ITS), and the 17S and 26S gene regions to construct
a phylogeny of the Convolvulus spp. in Macaronesia, and their closest
relatives. Forty-three species of Convolvulus were sampled, including
nine of the eleven species endemic to Macaronesia. The non-Macaronesian
species consisted of Convolvulus species from the Mediterranean and
western Asia, as well as some closely related species included as
outgroups. The aligned sequence data was used in a parsimony analysis
(Carine et al., 2004).

The topology of the consensus tree shows two major clades, one
consisting of perennial non-climbing species (mostly shrubs and
subshrubs), and the other composed of annual or perennial herbs and
suffrutescent plants. The Convolvulus species endemic to Macaronesia are
resolved as two distinct clades nested within these two groups (Carine
et al., 2004). These phylogenetic relationships imply that there were
two independent colonizations of Macaronesia from distantly related
lineages of Convolvulus (Carine et al., 2004). The erect shrubs (C.
caput-medusae, C. scoparius, C. floridus) resulted from one
colonization, while the climbers (C. canariensis, C. fruticuIosus, C.
lopezsocasi, C. massonii, C. glandulosus, C. volubilis) originated from
another. The analysis also shows that woodiness is plesiomorphic in the
erect shrubs, while it is derived in the climbers (Carine et al., 2004).

Just as in Tolpis, the example of Convolvulus shows that plants
with insular woodiness are not necessarily secondarily woody. Although
none of the species of Convolvulus were examined for paedomorphic
characteristics, the implications are clear. Intuitive assessments of
morphology used to determine relationships and the evolutionary origins
of woodiness, while useful, should be supplemented with additional
information and analyses. Phylogenetics, particularly molecular
phylogenies, provides a useful supplemental tool to anatomical and
morphological analyses.

F. Paedomorphosis with Primary Woodiness

Although a significant amount of work suggests a link between
paedomorphic wood and secondary woodiness, there is no required
connection between these two phenomena. In some studies, plants with
paedomorphic secondary xylem have been shown to be secondarily woody
through phylogenetic analysis. In others, the presence of paedomorphic
wood has been used to suggest, ipso facto, secondary woodiness (Lens et
al., 2009; Lens et al., 2005a). However, plotting the co occurrence of
scalariform perforation plates and Paedomorphic Type I rays on the
phylogeny of the Ericaceae shows that at least these two characteristics
of paedomorphic wood can also occur in the context of primary woodiness
(Fig. 13). The plots showing the evolution of woodiness in the taxa
related to Xanthorhiza and Mahonia (Figs. 4, 12) also demonstrate that
the relationship between paedomorphosis and secondary woodiness is not
straightforward.

Both of these lines of evidence suggest that paedomorphic wood
cannot be used as the sole basis for ascertaining the existence of
secondary woodiness. Since plants with paedomorphic woodiness can be
either primarily or secondarily woody, the evolutionary source of their
woodiness is best determined through phylogenetic analysis. Whether
primary or secondary woodiness exists should not be based on anatomical
or morphological analyses alone.

The wood anatomy of the Cactaceae also provides evidence that
paedomorphic characteristics are sometimes found in primarily woody
species. The Cactaceae is a very diverse family, all of which produce
some secondary xylem. The family includes trees, vines, dwarf plants
such as the globose cacti, giants such as the huge columnar cacti,
epiphytes, and geophytes. The wood anatomy of the family reflects this
structural diversity (Mauseth, 2006a).

All genera of the Cactaceae are woody in the sense that they have a
vascular cambium that produces secondary xylem and phloem. In some
cases, like the dwarf globose cacti that are only 2-3 cm in diameter,
they may not produce much secondary xylem, but some occurs in all genera
(Mauseth, 2006a). To meet the diverse structural needs of the species a
variety of types of secondary xylem are produced, including fibrous
wood, wide band tracheid wood, dimorphic wood, trimorphic wood, and
parenchymatous wood (Mauseth, 2004, 2006b; Mauseth & Plemons, 1995;
Mauseth & Plemons-Rodriguez, 1998; Mauseth & Stone-Palmquist,
2001). Both the shoot and the root may produce the same type of wood, or
wood type may vary by organ. Wood type may also change over time in the
same organ (Mauseth & Stone-Palmquist, 2001).

Fibrous wood, which contains vessels embedded in a matrix of
fibers, is predominant in genera with tall branching or columnar forms,
and in long scrambling plants. For example, fibrous wood is found in
Monvillea diffusa, Cereus hankeanus, and Pilosocereus lanuginosus
(Mauseth & Plemons-Rodriguez, 1998). This wood may be monomorphic in
both seedlings and adults, or it may be dimorphic. Monomorphic fibrous
wood is commonly found in tall woody species like Pereskia sacharosa
(Mauseth & Plemons-Rodriguez, 1998). In other genera fibrous wood is
only one stage in a dimorphic or trimorphic wood.

Wood with wide band tracheids is found in almost all genera of
Cactaceae (Mauseth, 2004, 2006b). This type of wood consists mainly of
vessels embedded in a matrix of tracheids. The tracheids are short and
barrel shaped, and have annular or helical secondary wall deposition
material projecting deeply into the lumen. The term "wide
band" refers to the appearance of the annular or helical
projections, which resemble bands (Mauseth, 2004).

Wide band tracheids offer a major adaptive advantage: the annular
or helical bands allow the tracheids to expand and contract with
changing moisture conditions. In this way the volume of each tracheid
can be matched to the amount of water it contains. Coupled with the fact
that these woods lack fibers, the presence of wide band tracheids allows
the stem to contract under dry conditions, and expand under wet. As a
result, the risk of cavitation is reduced. Tracheids with more rigid
cell walls have fixed volumes, and lack this advantage (Mauseth, 2004).

Wide band tracheid wood is monomorphic in some species, and is the
first form of wood produced in some dimorphic and trimorphic species
(Mauseth & Plemons, 1995). It is present as the monomorphic wood in
Astrophytum ornatum, Echinocactus knuthianus, and Blossfeldia
liliputana. These are small, globose or broad, columnar cacti (Mauseth,
2004; Mauseth & Plemons-Rodriguez, 1998).

In some dimorphic species, such as Buiningia aurea, Oreocereus
selsianus, and Vatricania guentheri, wide band tracheid wood is
produced, followed by more typical fibrous wood. In many of the
dimorphic species, secondary rays are not common, and are often narrow
(only 1-3 cells wide) when they occur. In other dimorphic species wide
band tracheid wood is followed by parenchymatous wood, with vessels
embedded in parenchyma. Species with this type of wood include
Echinopsis tubiflora, Gymnocalycium marsoneri, and Parodia maassii.
Secondary rays in these species are also narrow and sparse. In E.
tubiflora, there is delayed development of the secondary rays so that no
rays develop in the secondary xylem until the vessels are abundant. The
final type of dimorphic wood contains fibrous wood as the first stage,
followed by a second stage of parenchymatous wood. This is found in such
species as Hylocereus venezuelensis and Stephanocereus leucostele
(Mauseth & Plemons, 1995).

In trimorphic species wide band tracheid wood is followed by
fibrous wood, and then parenchymatous wood as a final phase. Trimorphic
wood occurs in Melocactus intortus, Melocactus bellavistensis, and
Arrojadoa braunii (Mauseth & Plemons, 1995).

Parenchymatous wood is not extremely common, but has been found in
16 species to date. It usually occurs as a stage in dimorphic or
trimorphic wood, in mostly short globose plants (Mauseth &
Plemons-Rodriguez, 1998; Mauseth & Stone-Palmquist, 2001). However,
the roots of species such as Acanthocereus sicariguensis, Corryocactus
megarhizus, and Copiapoa coquimbana have monomorphic parenchymatous wood
(Mauseth & Plemons-Rodriguez, 1998).

As the previous discussion indicates, there is a correlation
between wood anatomy and plant structure in Cactaceae. Adult species
with short globose structure tend to have wide band tracheid wood or
parenchymatous wood, or a dimorphic form containing both types. In these
plants cell turgor is sufficient to provide structural support. Species
with tall columnar adults or branching forms tend to have fibrous wood,
either throughout their entire life cycle or as the major portion of a
dimorphic wood. In many of these species, turgor pressure provides
adequate support while the plant is small. As the plant enlarges,
fibrous wood begins to be produced and provides greater support
(Mauseth, 2006b; Mauseth & Plemons, 1995; Mauseth &
Plemons-Rodriguez, 1998).

In general, monomorphic wood is found in species where the
structure of the adult is very similar to the young plant. Dimorphic
wood is usually found in species where there is a change in structural
form from seedling to adult. For example, species like Astrophytum
ornatum and Gymnocalycium oenanthemum have short and broad young shoots
whose turgor is sufficient to support the plant. Their juvenile wood
contains wide band tracheids. As they grow in height turgor provides
insufficient support, and they switch to producing fibrous wood (Mauseth
& Stone-Palmquist, 2001).

Both adult wood with wide band tracheids and adult parenchymatous
wood can be considered paedomorphic. In many of the plants with adult
parenchymatous wood the structure of the wood resembles that of the
primary xylem (Mauseth & Plemons, 1995). Both tissues consist of
vessels embedded in a parenchymatous matrix. In at least some genera the
primary xylem also possesses wide band tracheids (Mauseth & Plemons,
1995). In addition, in at least one species (Echinopsis tubiflora) there
is delayed ray development, another characteristic of paedomorphic wood.
In monomorphic wood these characteristics exist for the life of the
plant, while in dimorphic and trimorphic woods they exist for a
prolonged period before a different type of wood is produced. These
features, while somewhat different from those described by Carlquist
(1962), are paedomorphic because they occur in the primary and persist
into the secondary xylem.

As mentioned above, all Cactaceae have a vascular cambium that
produces secondary xylem and phloem. The ancestors of the family are
also thought to be woody, probably woody non-succulent trees or large
shrubs with fibrous wood. The sister group to the rest of the family,
Pereskia, possess monomorphic, fibrous wood (Edwards et al., 2005;
Manseth, 2006b; Manseth & Stone-Palmquist, 2001). It has also been
argued that the genera that possess either monotypic wide band tracheid
wood, parenchymatous wood, or these wood types as a prolonged stage in
dimorphic wood, are moving evolutionarily away from typical fibrous wood
towards less woodiness (Mauseth & Plemons, 1995; Mauseth &
Stone-Palmquist, 2001). It seems that these types of paedomorphic woods
are derived from woody ancestors in the Cactaceae.

Although research on wood anatomy in Cactaceae has not focused on
paedomorphosis, the presence of paedomorphic characters in these woods
provides another example of paedomorphic wood in species with primary
woodiness. Unless additional research provides support for an exclusive
link between paedomorphic wood and secondary woodiness, it is better to
use phylogenetic analysis in conjunction with wood anatomy to gauge
whether primary or secondary woodiness exists.

The fact that paedomorphic wood is produced by the plant when
turgor is sufficient for its structural needs supports Carlquist's
hypothesis that paedomorphic wood represents a relaxation of selection
for mechanical strength (Carlquist, 2001). It may be that that
paedomorphic wood arose through a heterochronic shift in flowering time
when previously immature plants became capable of flowering. With no
need for additional structural support, the plants continued to produce
juvenile wood. Thus, juvenile features of the secondary xylem are the
only ones expressed by the genes of these plants, while the genetic
program for producing fibrous wood has been repressed (Mauseth, 2004;
Mauseth & Stone-Palmquist, 2001).

G. Paedomorphic Rays and Raylessness

Carlquist (1962) first suggested that raylessness was associated
with paedomorphosis in his classic paper on paedomorphosis, and
developed his ideas more fully in relation to insular species of
Plantago (Carlquist, 1970). In this paper he noted the correlation
between rayless and other paedomorphic characters, after which he and
others began to use raylessness as an indication of paedomorphosis (Lens
et al., 2007).

A brief justification for considering raylessness a paedomorphic
character was presented in Carlquist (2009). Here he says [Raylessness
occurs] "in a small number of eudicot species, and in some of these
rays eventually form. The fact that ray formation is delayed is thus the
expression of juvenilism." To understand this argument we must
first understand how multiseriate and uniseriate rays form at the
beginning of secondary growth (Barghoorn, 1940, 1941a, b).

At least in the species Barghoorn (1940, 1941a, b) investigated,
multiseriate rays form from the interfascicular regions of the primary
plant body (the primary rays), as radial continuations of these regions.
Uniseriate rays form only in the fascicular regions, after secondary
growth has begun. Additional multiseriate rays may also form in the
fascicular regions. As secondary growth continues, multiseriate rays may
become divided to form smaller rays, and uniseriate rays may disappear.
These processes occur through the replacement of ray initials with
fusiform initials, and may take place gradually (Barghoorn, 1941b). As
the ray initials lengthen, the resulting ray cells become more upright.
It should be possible to follow the disappearance of rays in an
individual in serial tangential sections, which should show a gradual
lengthening of the ray cells before they are replaced by tracheary
elements, or fibers. The presence of rays with diffuse lateral borders
in Calepina irregularis (Brassicaceae) may be a stage in this process
(Schweingruber, 2006).

In rayless species the transformation of ray to fusiform initials
takes place as soon as secondary growth begins in the interfascicular
regions (Barghoorn, 194 lb). The cells of the primary rays are
immediately converted to fibers or tracheary elements. In the fascicular
regions uniseriate rays simply never form. If we consider only the
fascicular regions of rayless species, the juvenile process of ray
formation is delayed, or never occurs. Raylessness is thus an expression
of paedomorphosis, as is the possession of only uniseriate rays. If we
consider the interfascicular region, the conversion of ray to fusiform
initials, which usually occurs later in growth, now occurs at the
beginning of secondary growth. This process results in a form of
peramorphosis (acceleration), the expression of mature characteristics
in a juvenile. So, somewhat paradoxically, rayless wood can be seen as
both paedomorphic and peramorphosis depending on whether one focuses on
the fascicular or interfascicular regions.

In summarizing these mechanisms it is important to note that
Barghoon's (1940; 1941a; b) results are based on a limited sample
of more or less typical woody species. As far as we are aware, his
results have not been corroborated on species with a wider range of
secondary growth types (Krumbiegel & Kastner, 1993; Schweingruber
& Landolt, 2005-2008).

If the absence of rays is a functional adaptation to a reduced need
for lateral transport, the presence of solely uniseriate rays with
upright cells (Paedomorphic Type III rays) might be a transitional stage
between multiseriate rays (Paedomorphic Types I and II) and raylessness.
Woods such as those of Archeria traversii and A. fasciculata (Ericaceae)
(Meylan & Butterfield, 1978) have mainly uniseriate rays consisting
of upright cells, and are shrubby (up to 5 m tall). Their woods at least
superficially resemble those found in truly rayless species like those
of Veronica section Hebe (Scrophulariaceae) (Meylan & Butterfield,
1978; Philip Garnock-Jones, personal communication). If Barghoorn
(1941b) is correct that raylessness results from the substitution of
fusiform shaped fibers for ray cells, then it seems reasonable that the
formation of upright parenchyma cells in uniseriate rays is a
transitional stage between multiseriate rays and the rayless condition.

H. The Concept of Woodiness in Relationship to Plant Growth Forms

The concept of a woody plant is central to the theory of
paedomorphosis, and to this paper (Carlquist, 1962). At the outset we
tried to restrict our use of the term woody to refer to the possession
of fibrous wood, but our work on Coreopsis has shown the futility of
this restriction. If we learn only one thing from Carlquist's
voluminous work on xylem evolution, it should be that the concept of a
woody plant is badly in need of refinement.

The fact that woody plants can readily evolve from nonwoody
ancestors in a variety of environments shows that the xylem evolution is
very labile. Not only does secondary xylem occur in numerous lineages,
but the organ in which it evolves may vary. It may be absent from the
stem, but found in the root system (Liu & Zhang, 2007; von Arx &
Dietz, 2006). For example, in many dwarf Opuntias (Cactaceae) only
fascicular cambium is formed in the stem. This cambium produces only a
few vessels and some xylary parenchyma. Yet the roots of these species
form large amounts of wood (James Mauseth, personal communication). The
degree of woodiness may also vary within the same organ over time.
Cereus aethiops (Cactaceae) serves as an extreme example of this
phenomenon. Its shoot and root systems initially produce typical fibrous
wood. However, some of its roots become greatly enlarged and switch to
producing parenchymatous wood, while its slender roots continue to
produce fibrous wood (Mauseth & Stone-Palmquist, 2001).

Carlquist (1962) cites a number of examples of annuals with flat
age-on-length curves to bolster his case that these curves can be
considered equivalent to the descending portions of normal age-on-length
curves (Fig. 1). The implication is that even annuals can possess
significant amounts of secondary xylem.

In the Ericaceae, the taxa Chimaphila umbellata and Pyrola
rotundifolia have traditionally been considered nonwoody (Kron et al.,
2002). However, both have reasonable amounts of typical fibrous wood at
the root/shoot junction (Schweingruber & Landolt, 2005-2008), and
could be considered woody according to conventional definitions. Even
the annual Linum usitatissimum (flax; Linaceae) produces significant
amounts of secondary growth (Bowers, 1996). These examples show that
woodiness is not an all-or-nothing phenomenon, but rather occurs along a
continuum (Table 1) (Schweingruber & Landolt, 2005-2008).

This point is reinforced by the work of Krumbiegel and Kastner
(1993), who describe seven forms of secondary growth in the shoots (and
two in the roots) of annuals: (1) Secondary growth restricted to the
vascular bundles (two subtypes). (2) Fascicular and interfascicular
cambia formed, but secondary lignified cells only produced opposite the
primary bundles. (3) Fascicular and interfascicular cambia formed, with
secondary lignified cells produced in both regions (two subtypes). (4)
As in type 3, but with growth occurring in the fascicular regions prior
to the formation of a complete cambial ring (two subtypes). (5) A
complete, active cambial ring is present at the beginning of shoot
differentiation so that it is difficult to determine the boundary
between primary and secondary growth (two subtypes). (6) Secondary
growth produced by successive cambia. (7) Fascicular cambia present, and
interfascicular secondary growth produced by cell divisions in the inner
layer of the cortex.

These seven types of secondary growth produce plants that, though
they all have secondary xylem, are not all woody, at least not in the
conventional sense (Krumbiegel & Kastner, 1993). Bower's (1996)
use of the term arborescent instead of woody is an implicit recognition
of this problem. Although he does not discuss these terms in detail,
Bower (1996) distinguishes herbaceous from arborescent, not from woody.
This pairing of terms puts the emphasis on the growth form, not the
presence or absence of secondary xylem.

These observations also call into question the concept of
herbaceousness. Currently, the term herbaceous is used in two senses. In
addition to referring to stems without secondary growth, it is used to
refer to plants that die back at the end of the growing season. If an
herb like Trifolium alpinum can have up to 50 annual growth increments
(Schweingruber & Poschlod, 2005), then the equation of herbaceous
with a lack of secondary growth must be spurious. Even annuals such as
Capsella bursa-pastoris and Cardamine flexuosa (among many others, Table
l) are capable of significant amounts of secondary growth. When annuals
germinate in the winter and flower in the spring, two growth rings are
often present (Schweingruber, 2007a; Schweingruber & Landolt,
2005-2008), producing even more terminological confusion.

That so called herbaceous plants can produce secondary growth is
also evident from recent work on tobacco and Arabidopsis (Oh et al.,
2003). Neither of these plants normally produces secondary xylem.
However, under special conditions they can both be induced to do so. For
example, when Arabidopsis has its inflorescences removed and is grown
under short days it produces rayless secondary xylem (Oh et al., 2003).
In addition, mutations in the genes SOC1 and FUL cause considerable
amounts of secondary xylem to be produced under both long and short
photoperiods (Melzer et al., 2008). Clearly, even herbaceous annuals
like Arabidopsis must have the genes for secondary growth. All that is
needed to trigger the development of secondary xylem is a change in
environmental conditions. This, in turn, alters gene expression and
produces a woody tissue (Groover, 2005).

An interesting question is whether any aspect of the genetics
required for the production of normal wood has been lost in groups with
both herbaceous and woody members. The species of Ranunculus that have
been studied in Europe lack all secondary growth (Table 1)
(Schweingruber & Landolt, 2005-2008), yet occur nested within woody
taxa (Fig. 4). It would be interesting to know if any of its species has
retained the genes necessary for the production of woody tissue. This
question could be approached by comparing the part of the genome
responsible for xylogenesis from a plant like Populus, whose genome has
been completely sequenced, to its genetic counterpart in Ranunculus.

These observations strongly suggest that the meaning of the term
herbaceous must either be radically changed so that it includes plants
with secondary growth, or restricted to the relatively few plants that
truly have no secondary growth. The vast majority of herbs are not, in
fact, herbaceous (Bowers, 1996; Krumbiegel & Kastner, 1993).

The use of the terms herbaceous for plants whose above ground parts
do not perennate, and woody for those that do, is also problematical
because these usages ignore the form with which the die back occurs.
Many plants perennate by retaining buds at, or just above, the soil
surface. These plants are neither traditionally herbaceous, nor woody.
Many of the plants in which Carlquist describes paedomorphic wood fit
into this class. Although some of them can be described as
suffrutescent, this term, like herbaceous, conflates growth form with
the presence of wood. Suffrutescent plants are shrubs with stems that
are woody only at the base. When the full range of growth forms is taken
into account, we have to agree with Fritz Schweingruber (2007b) when he
says "Anatomical characteristics would appear to have little
influence on a plant's growth form." It would be better to use
terms that do not conflate these seemingly unrelated aspects of plant
form.

Raunkiaer proposed just such terms in his classification system of
plant life forms (Raunkiaer, 1904, 1934). Phanerophytes have buds that
survive unfavorable seasons on aerial, negatively geotropic shoots.
Trees are typical phanerophytes. In chamaephytes the surviving buds are
born on shoots very close to the ground. Hemicryptophytes bear their
perennating buds at the soil surface, while in cryptophytes the buds are
buried in the soil. The final category is therophytes (annuals): plants
that pass unfavorable seasons as seeds (Raunkiaer, 1904, 1934). Later
work by other authors has extended the classification system to
non-vascular and tropical plants (Ellenberg & Mueller-Dombois,
1967). None of these classifications are routinely used in discussions
of plant anatomy, though they are clearly applicable (Schweingruber,
2006; Schweingruber, 2007a; Schweingruber & Landolt, 2005-2008).

Adopting Raunkiaer's terminology would allow the
disambiguation of the terms woody and herbaceous. These terms could then
be used for the presence and absence of secondary growth, respectively.
Growth forms would be described by an independent set of terms that
would more accurately reflect how plants perennate. Disambiguating these
terms would do away with hybrid categories such as "woody
herbs." This category makes as much sense as the seldom used
"herbaceous trees."

VI. Conclusions

The theory of paedomorphosis in the secondary xylem is an
explanation for certain character and character state trends found in
the secondary xylem of plants that often have shrubby, suffrutescent,
pachycaulous, or lianoid growth forms. These characteristics include a
decreasing, or stable, length of vessel elements as the xylem ages; the
presence of scalariform perforation plates; scalariform or
pseudoscalariform lateral wall pitting; and the absence of rays, delayed
ray development, or rays consisting solely of upright cells. Earlier
reports that paedomorphic wood can be identified by the presence of
libriform fibers and simple perforation plates are no longer considered
valid.

Many species described as having paedomorphic wood are found on
islands, so the concept of insular woodiness is closely associated with
paedomorphosis and with the occurrence of secondary woodiness. Although
paedomorphosis is often associated with secondary woodiness, there is no
necessary association between the phenomena, and secondary woodiness
cannot be identified based solely on the possession of paedomorphic
wood. The possession of secondary woodiness is best determined following
phylogenetic analysis.

All three species investigated here were found to have a degree of
paedomorphosis, with Coreopsis gigantea having the largest number of
paedomorphic characteristics. Mahonia bealei has only a single
paedomorphic character (a flat age-on-length curve), while Xanthorhiza
simplicissima has an intermediate number. Repeated measures of vessel
element lengths across the secondary xylem of a single mature stem of C.
gigantea produced both negatively sloped and flat age-on-length curves,
suggesting that there may be no statistical difference between
descending and flat curves. Plotting the character states woody and
nonwoody on phylogenetic trees containing these taxa shows that
Coreopsis gigantea is secondarily woody, while the ancestral state of
the other two species is equivocal. However, the validity of these
reconstructions is at least somewhat questionable due to the often
unreported occurrence of secondary growth in "herbaceous"
plants.

The presence of paedomorphic secondary xylem has been suggested as
a response to relaxed selection for mechanical strength (Carlquist,
2001). The xylem structure of the three species investigated here is
consistent with this hypothesis. Their apparent mechanical strength is
inversely correlated with their degree of paedomorphosis. Given this
inverse relationship, it is possible that the presence of paedomorphic
wood may be more of a consequence of a plant's growth habit than
its possession of secondary woodiness. The occurrence of paedomorphic
wood in primitively woody species would seem to obviate any causative
relationship between paedomorphosis and secondary woodiness. Additional
research is needed to explore the link between paedomorphosis and
mechanical strength by examining the correlation between the occurrence
of paedomorphic xylem, stem strength, and growth habit in a phylogenetic
context.

The frequent occurrence of secondary growth in plants such as
annuals and suffrutescent shrubs calls into doubt the use of terms such
as woody and herbaceous to distinguish plants with, and without
secondary xylem. These terms conflate two independent aspects of plant
form, growth habit and the presence of secondary xylem. If these terms
are to remain useful, their use must be restricted to one of their two
meanings. Since better alternatives exist for the description of growth
forms, it would be better to restrict them to the description of the
presence or absence of secondary growth. The term herbaceous should only
be used for plants that truly lack secondary growth, such as species of
Ranunculus sampled here.

VII. Acknowledgements

We are grateful for the assistance we received while carrying out
this research. Pete Diamond (formerly with the NC Zoo), assisted with
the collection of Xanthorhiza simplicissima, Holly Forbes (UC Berkeley
Botanical Garden), provided specimens of Coreopsis gigantea, and Matina
Kalcounis-Ruppell (UNC Greensboro) allowed the collection of Mahonia
bealei on her property in Greensboro, NC. We are especially grateful to
Sherwin Carlquist for his many comments, and for his encouragement
during the research and writing process. Fritz Schweingruber, Frederick
Lens, James Mauseth, David Remington, Pieter Baas, John Sperry, Philip
Garnock-Jones, Katherine Kron, Norm Douglas, and Jian-Yong Wu all
provided data or comments that greatly improved the manuscript. Fritz
Schweingruber provided access to unpublished data on wood anatomy of the
Brassicaceae, some of which is presented in Table 1. This paper is
based, in part, on a Thesis submitted by the first author to the
University of North Carolina at Greensboro in partial fulfillment of the
requirements for an M.S. degree. We thank David Remington and Dennis
LaJennesse for providing guidance during this process. Funding for the
research was provided by grants from the Student Research Support Fund
of the Department of Biology, UNCG. All responsibility for the opinions
expressed here rests with the authors.

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(2) For the purposes of this discussion we have amended our
definition of wood as fibrous secondary xylem to include the highly
parenchymatized secondary xylem of Coreopsis gigantea. We return to this
issue in the last section of the Discussion.